composition and oxidation stability of sae 15w-40 engine oils

Tribology International 34 (2001) 127–134www.elsevier.com/locate/triboint

Composition and oxidation stability of SAE 15W-40 engine oils

J. Cerny*, Z. Strnad, G. SeborDepartment of Petroleum Technology and Petrochemistry, Institute of Chemical Technology, Technicka 5, 16628 Prague, Czech Republic

Received 28 February 2000; received in revised form 3 November 2000; accepted 4 December 2000

Abstract

The modified IP 48 procedure was used for testing the oxidation stability of engine oils for ignition engines of the SAE 15W-40 and minimum API SG/CD specifications. Eight engine oils, such as Agip, BP, Esso, Mogul, O¨ MV, Petro-Canada, Shell, andTotal, were collected and oxidised. Oils were also fractionated by using chromatographic methods, and group composition of baseoils, amount of VI improvers and other additives were determined. Several differences in the oxidation stability were found betweenthe collected oils. Viscosity characteristics, carbon residue, acid number, amount of pentane insolubles and mass losses during thetest were chosen as parameters for evaluation of the oxidation stability of the oils. The most stable engine oil was blended froma hydrocracked base oil whereas most of the other oils were based on the solvent refined oils. 2001 Elsevier Science Ltd. Allrights reserved.

Keywords:Engine oils; Chemical composition; Oxidation stability

1. Introduction

In the literature, there are a great number of referencesabout oxidation and thermooxidation stability of lub-ricating oils. Unfortunately, many of them are ambigu-ous or even contradictive [1–11]. Oxidation stability oflubricating oils is largely dependent on the oxidation testused [4]. There are several standardised laboratory tests,which are carried out at different temperatures, mostlyin the temperature interval from 95°C to 200°C. Suchan interval is very wide as a temperature of oxidationtest, its duration and some other parameters are mostlikely the key points for assessment of oxidation stabilityof lubricating oils.

Oxidation of oils undergoes three oxidation stages andthe stage in which the oxidation test is stopped is decis-ive for evaluation of oil ageing [4].

The first oxidation stage is the inhibition period. Inthis stage, properties of the oil are relatively stable andthe oxidation extent is very small. Duration of the inhi-bition period is predominantly affected by temperatureand by the concentration of antioxidants. This stage ter-

* Corresponding author. Fax:+420-2-2431-0481.E-mail address:[email protected] (J. Cerny).

0301-679X/01/$ - see front matter 2001 Elsevier Science Ltd. All rights reserved.PII: S0301-679X(00 )00150-X

minates after depletion of synthetic antioxidants and oxi-dation of oil starts to be more pronounced.

The second oxidation stage is the breakdown stage, inwhich oxidation is not influenced by synthetic antioxi-dants. The most pronounced effect on oxidation rate canbe seen in a composition of the oil and in concentrationof natural oxidation inhibitors [1,4,5,10]. Increased oxi-dation rate also deteriorates properties of the oil.

The final oxidation stage is characterised by a slowoxidation rate due to a high oil viscosity. Oil contains ahigh concentration of oxidation products which can bepartly polymerised and high oil viscosity then limits theaccess of air or oxygen into oil by a limited rate of dif-fusion [4,5].

An important factor in comparison of results from dif-ferent oxidation tests is oxidation temperature and thestage in which the oxidation test is stopped [4]. Thatlargely affects conclusions made from evaluation oftests. The amount of saturated hydrocarbons is a keyparameter for stability of oils which are oxidised at lowtemperatures and/or for tests which are stopped duringthe inhibition stage, i.e. before antioxidants weredepleted [4–7,12]. That is most likely due to the highdissociation energy of C–H bonds in saturated hydro-carbons. In high temperature tests which are stopped inthe breakdown stage there is a very important parameter,the aromatics and sulphur contents in oils, as the sulphur

128 J. Cerny et al. / Tribology International 34 (2001) 127–134

aromatic compounds can act as natural inhibitors duringthe breakdown stage [1,4,10,13,14].

Oxidation stability is one of the serious properties offormulated oils influencing the oil quality within the oildrain interval. However, current engine tests included inspecifications such as API SJ/SH and/or ILSAC GF-2evaluate preferably piston deposits, engine sludge andvarnish. These evaluation parameters are not directlyassociated with the oil oxidation and many other effectscan play a role in formation of sludges and deposits.Moreover, there are some references that ageing duringthe engine tests, e.g. Sequence VIA, should be moreextensive to be realistic [15]. For example, depletion ofantioxidant capacity after the Sequence VIA test wasfound to be about 20%, whereas that after 6700 km mile-age accumulation in vehicle testing was greater than50% [15].

This work is aimed at testing the oxidation stabilityof fully formulated engine oils for spark ignition enginesof the SAE 15W-40 specification. The high-temperaturelaboratory standard test IP 48 was modified for that pur-pose by prolongation of the oxidation time. Evaluationof the oxidation tests was made by using the standard-ised methods required by the IP 48 method, and severalother parameters of the fresh and oxidised oils were alsomeasured. Another goal of the work was to compare oxi-dation stability of several engine oils of the SAE 15W-40 specification from important producers and to makesome conclusion about oxidation stability relative tocomposition of base oils.

2. Experimental

2.1. Samples

Eight samples of the SAE 15W-40 engine oils forspark ignition engines were collected. Their alphabeticallist and corresponding specifications are presented inTable 1. All oils were obtained from official distributorsin the Czech Republic and represented the highest qual-ity SAE 15W-40 oils of each producer.

Table 1SAE 15W-40 engine oils tested and their specification

API ACEA CCMC

Agip F1 Super SJ/CD A3-96, B2-96 G4, PD2BP Visco 2000 SJ/CF A2-96, B2-96 G4, PD2Esso Uniflo SJ/CF A2-96, B2-96Mogul Felicia SH/CF A3-96, B3-96 G4, PD2OMV Control SG/CF-4 G4, D4, PD2Petro-Canada Gold HTX 600 SG/CF-4Shell Helix Super SH/CDTotal Quartz 5000 SH/CF A2-96, B2-96

2.2. Fractionation of formulated oils

Composition of oils was assessed by a combinationof a size exclusion and adsorption chromatography. Pre-parative size exclusion chromatography was performedon a glass column 10 mm i.d. and 1000 mm long thatwas filled with a polystyrene–divinylbenzene gel Bio-Beads SX3. A mixture of toluene and ethylalcohol 9:1v/v was used as a mobile phase. Two fractions wereobtained after fractionation, i.e. fraction of polymeradditives and the rest of oil. Separation of polar additivesand polar oil compounds, and fractionation of the baseoil was done by adsorption chromatography on a glasscolumn 10 mm i.d. and 1000 mm long filled with silicagel and neutral alumina according to a modified pro-cedure of Sawatzky [16]. Briefly, five fractions wereobtained after fractionations, i.e. saturated hydrocarbons,three fractions of aromatics according to the ring num-ber, and one fraction of polar compounds.

2.3. Modification of the IP 48 oxidation test

The IP 48 oxidation test is usually used for testingbase lubrication oils. To reach a sufficient oxidationextent of formulated oil the IP 48 method was modifiedby a prolongation of the oxidation time from two to four6 h oxidation cycles (total oxidation time was 24 hours).Other conditions of the IP 48 were maintained, i.e. tem-perature of 200°C, air flow rate of 15 l h21. A commer-cial apparatus from ANALIS was used for oxidation ofall oils. A suitability of the modified IP 48 test for for-mulated oils is discussed below.

2.4. Evaluation of the oxidation tests

According to the IP 48 standard, oxidation testsshould be evaluated by measuring kinematic viscosity at40°C (ASTM D445) and Ramsbottom carbon residue offresh and oxidised oils. As suitable apparatus for deter-mination of the Ramsbottom carbon residue was notavailable, the Micro Carbon Residue Test (MCRT)according to ASTM D4530 was determined instead.

129J. Cerny et al. / Tribology International 34 (2001) 127–134

Additional analyses of fresh and oxidized oils were doneby measurement of viscosity at 100°C and calculationof viscosity index (ASTM D2270), by acid numberdetermination by potentiometric titration (ASTM D664),and by pentane insolubles (GOST 11063-64).

FTIR spectra of oils were accumulated at 2 cm21 res-olution on the Nicolet 740 spectrometer. Oils were meas-ured in a substance in a 0.1 mm KBr cell or in the formof film between two KBr discs in the 4000–400 cm21

range. Sixty-four scans were acquired. The absorptionbands of carbonyls at about 1700 cm21 were evaluatedto assess the oxidation extent.

3. Results and discussion

3.1. Composition of SAE 15W-40 oils

As mentioned in the Experimental section, compo-sition of the oils was analysed by size exclusion andadsorption chromatography, both in a preparative mode.Size exclusion chromatography led to a determination ofthe amount of polymer additives in oils, and adsorptionchromatography gave insight into a composition of baseoil used for production of the oils. Results are summar-ised in Table 2.

As can be seen from Table 2, the amount of polymeradditives was very similar in almost all oils analysed andlay between 7 wt% and 8 wt%. Some exceptions werefound for the oils C and H, in which the amount of poly-mer additives was above or below this range, respect-ively. As to the absolute values of the polymer additivesamount, it can be somewhat overestimated due to themechanism of the size exclusion chromatography. Someof the most polar low molecular weight additives elutefrom Bio-Beads gels very quickly and can be elutedtogether with the polymeric material. It was especiallythe case of some metal carboxylates, most likely overb-ased alkylsalicylates, which were detected by FTIR spec-troscopy in almost all polymer fractions. Detection of

Table 2Chromatographic fractionation of the SAE 15W-40 oils

Polymer additives Composition of base oil+low MW additives (wt%)in oil (wt%)

Saturates Monoaromatics Diaromatics Polyaromatics Polar compounds

Oil A 8.0 72.5 17.7 2.7 1.6 5.5Oil B 8.2 71.4 15.2 2.7 2.9 7.8Oil C 10.4 69.6 16.9 3.3 3.2 6.9Oil D 8.1 67.3 18.3 4.2 3.4 6.8Oil E 7.2 83.7 10.9 1.7 1.6 2.1Oil F 7.4 84.7 7.2 1.4 0.6 6.1Oil G 8.0 93.6 2.9 0.5 0.3 2.8Oil H 6.4 73.1 16.4 3.3 2.3 4.8

alkylsalicylates was supported by absorption bands near1560 cm21 (carboxylate ions) and 1390 cm21

(carbonates). Very intensive and well resolved absorp-tion bands of alkylsalicylates were found in the FTIRspectrum of the polymer fraction of the oil C and thatmight cause an overestimation of the amount of VIimprovers in that oil (see Table 2). Very similar was thecase of alkylphenolates which were also detected in theFTIR spectra of the polymer fraction. (C–O groupsaround 1230 cm21). Total amount of alkylsalicylates andalkylphenolates separated in the polymer fraction can beassessed as about 2 wt% to 3 wt%, showing thus thatthe real concentration of viscosity modifiers in the oilswas about 5 wt% to 6 wt%.

Qualitative evaluation of the FTIR spectra of the poly-mer additives gave evidence that VI improvers in almostall oils were either olefin copolymers or hydrogenatedstyrene–diene copolymers. Distinction between thesetwo types of VI improvers is very difficult by FTIR.Polymethacrylate polymers were only detected in one ofthe oils, namely the oil G. It was the only oil with sig-nificant absorption bands near 1730 cm21 and 1150cm21, which are typical for methacrylates.

Further fractionation of the oils free of polymer addi-tives gave results also shown in Table 2. Group compo-sition of the hydrocarbon matrix of oils allowed a com-parison of base oils used for production of formulatedoils. In the fractions of polar compound, there were con-centrated some polar compounds of the base oils andalmost all low molecular weight additives. A purity ofthe polyaromatic fractions was checked by FTIR spec-troscopy and no remarkable bands of additives wereobserved in the FTIR spectra. It is thus believed thatalmost all additives left in oil after the size exclusionchromatography were concentrated in the fractions ofpolar compounds.

As to the composition of base oils, there were twotypes of base oils from which the engine oils were pro-duced. For five of the engine oils, the composition oftheir base oils was very similar to typical solvent refined


oils with a content of saturated hydrocarbons about 70wt%. The other three engine oils (Oils E, F, and G) hadmuch lower content of aromatic hydrocarbons and thisgives evidence that a hydrocracked oil was used for theirproduction. Content of total aromatics of the Oil E was14.2 wt%. This value is too high for most hydrocrackedoils, and the amount of polyaromatics is also increased.It can therefore be deduced that the base oil for a pro-duction of the Oil E was mixed from hydrocracked andsolvent refined oils. Another possibility is the use ofmoderately hydrocracked oil only. Composition of thebase oil of the Oil F is typical for hydrocracked oils, butnot the VHVI grade. Content of total aromatics did notexceed 10 wt% and the amount of polyaromatics wasalso very low. Finally, the base oil of the Oil G wasclearly hydrocracked oil of the VHVI grade. Content oftotal aromatics was about 3.7 wt% and most of thatamount can originate from the solvent oil of the additivepackage. The use of VHVI oil for production of the OilG was also reflected in a high viscosity index of the oil(see below).

Special attention should be paid to the amount of polarcompounds (Table 2). From our experience, the amountof polar compounds in most solvent refined oils is in therange from 3.5 wt% to 5.0 wt%. It can be assumed fromthe amount of polar compounds in Oils A, B, C, D, andH that the concentration of pure oil additives, other thanVI improvers alkyl salicylate and alkylphenolates, in theoils was in the range from 2 wt% to 3 wt%. This amountof additives corresponds well to the concentration ofpolar compounds in Oils E and G made from hyd-rocracked base oils. The content of natural polar com-pounds in hydrocracked base oils is indeed very low oreven negligible and the polar compounds of the Oils E,F, and G were mostly the oil additives.

Together with metal carboxylates and alkylphenolatesdetected in the polymer fractions, the total amount ofadditives in the oils was thus at the level of 4 wt% to 6wt% in addition to about 5 wt% to 6 wt% of viscositymodifiers. The amount of polar compounds in the Oil Fwas found to be relatively high thus showing anincreased concentration of additives.

3.2. Modification of the IP 48 oxidation test

High temperature oxidation test IP 48 is a suitable testfor engine oils as the oil in cylinders and on piston ringsis exposed to temperatures above 200°C. However, thetest is not recommended for oils formulated with ashforming additives. The second limitation of the test isthat the mass losses of the oil during test should notexceed 10 wt%. Both limitations are probably associatedwith the problems with a viscosity measurement.Decomposition of some ash forming additives can leadto a formation of solid and/or insoluble products that candisturb the oil flow through the capillary of the viscos-

imeter. High mass losses of volatile oil components canlead to a digestion of the oil tested and to a viscosityincrease which was not caused by oxidation.

Despite the limitations described above, an effort tomodify the IP 48 test for oxidation of formulated oilswas made with the Oil E and results are shown in Fig.1. All conditions of the IP 48 test were maintainedexcept the oxidation time which was variable from 12to 30 hours in 6 h oxidation cycles. Similar tests weredone with nitrogen instead air to check the effect of oildigestion due to mass losses. Although the mass lossesof the oil at the end of the test exceeded even 20 wt%in both runs, it is clear from Fig. 1 that the viscositychange due to oxidation was significantly greater thanthat in the blind experiment with nitrogen blowing. Itwas believed that the difference in the viscosity valuesgives a satisfactory space for an evaluation of oxidationseverity. The Oil E oxidised for 30 hours was also ana-lysed for the amount of pentane insolubles, and theirconcentration was found to be negligible. That led us toa conclusion that the modified IP 48 oxidation test canbe used for an assessment and comparison of oxidationstability of the SAE 15W-40 engine oils. Four 6 h oxi-dation cycles were chosen as a suitable oxidation time.

3.3. Oxidation of engine oils of the SAE 15W-40specification

Oxidation tests were performed according to themodified IP 48 procedure described above. To meetspecifications of the IP 48 test, oxidation stability of oilswas evaluated by determining a kinematic viscosity at40°C and the amount of carbon residue (MCRT). Inaddition, kinematic viscosity at 100°C, mass losses dur-ing the test, amount of pentane insolubles, and acid num-ber were used as complementary evaluation parameters.FTIR spectra of fresh as well as oxidised oils werealso taken.

Viscosity characteristics of fresh oils were very simi-

Fig. 1. Viscosity changes of the Oil E during air oxidation and nitro-gen blowing.


Table 3Viscosity characteristics of engine oils of the SAE 15W-40 specifi-cation

Viscosity at 40°C Viscosity at 100°C VI

Oil A 107.88 13.91 129Oil B 106.22 14.03 133Oil C 107.93 14.36 136Oil D 110.25 14.97 141Oil E 99.19 13.94 143Oil F 110.95 15.09 142Oil G 106.52 15.70 157Oil H 105.73 14.30 138

Fig. 2. Increase in oil viscosity at 40°C after oxidation.

lar in almost all cases and are presented in Table 3. Vis-cosity at 40°C ranged mostly from 105 mm2 s21 to 111mm2 s21, the only exception was the Oil E whose vis-cosity was below 100 mm2 s21. Viscosity at 100°C wasabout 14 mm2 s21 except the Oils D, F, and G whoseviscosities were slightly higher with a consequence of acorrespondingly increased viscosity index.

Whereas there were only minor differences betweenthe fresh oils, after the oxidation test the viscositycharacteristics differed significantly. The viscositychanges after oxidation are shown in Figs. 2–4, in which

Fig. 3. Increase in oil viscosity at 100°C after oxidation.

Fig. 4. Change in viscosity index of oils after oxidation.

the oils are ordered according to increased changes.Remarkable differences were found for the viscositychanges at 40°C. A percentage increase of viscosityranged from 7% to 90% (Fig. 3), the lowest increaseexhibited the Oils F and C. Viscosity increase at 100°Cwas somewhat lower than that at 40°C (Fig. 4). Afteroxidation of the Oil F even a decrease in the viscositywas found. A very small viscosity increase was againfound for the Oil C.

Evaluation of oxidation stability of oils by viscositymeasurement is affected by a thermal stability of vis-cosity modifiers. Their degradation during thermal andoxidative ageing (C–C bond breaking and lowering themolecular weight) can lead to a viscosity decrease whilelow or moderate extent of oil oxidation proceeds. Thateffect is more pronounced at higher temperatures of vis-cosity measurements. During oil use in engines, vis-cosity increase due to oxidation is additionally compen-sated for by a mechanical shear-down of polymers andfuel dilution. In fact, viscosity of used oils is frequentlylower than that of fresh oils.

Figure 4 shows the changes of viscosity index afteroxidation. The viscosity index of each oxidised oil waslower than that of the fresh oil. However, the percentagedecrease in VI oscillated mostly in a relatively narrowrange between 15% and 20%. It is therefore believedthat a change of VI during a high temperature oxidationis caused by a thermal stability of viscosity modifiersrather than by oxidative ageing of oil.

The amount of carbon residue after oxidation of oilsreflects thermal rather than oxidation changes in oils andit is most likely associated with the amount of aromaticsand other carbon forming compounds such as some oxi-dation products. From that viewpoint it can be under-stood that the lowest increase in carbon residue wasfound after oxidation of the Oils F and G which con-tained the lowest content of aromatics, Fig. 5. Carbonresidue of individual fresh oils was determined between1.1 wt% and 1.6 wt%.


Fig. 5. Increase in the amount of carbon residue after oxidation ofoils.

Weight losses of oils after the oxidation test IP 48 areshown in Fig. 6 and relatively large differences betweenthe oils were found. Weight losses for the individual oilslay in the range between 10 wt% and 20 wt%, the lowestwere again found for the Oils F and C. That gives to theoils a good potential for low consumption during theiruse in engines. Besides the volatility of the base oil,mass losses of oils during the oxidation test are mostlikely also influenced by an oxidative deterioration ofoils, which can lead to a formation of volatile low mol-ecular oxidation products. It was additionally provedduring oxidation tests of oils of another viscosity speci-fication. Oxidation stable oils showed low mass lossesduring test, and there was no correlation with volatilelosses according to Noack.

Pentane insolubles are most likely formed by a con-densation of polar oxidation products. Their amount inoxidised oils is shown in Fig. 7. For half of the oil theamount of pentane insolubles was almost negligible.However, the second half of the oxidised oils containeda significant amount of pentane insolubles that in thecase of Oils D and A reaches the values of 4 wt% and

Fig. 6. Mass losses of oils after oxidation.

Fig. 7. The amount of pentane insolubles in oxidised oils.

6 wt%, respectively. These oils also showed a highincrease in the amount of carbon residue. That can havea detrimental effect on the cleanliness of some of theengine parts. However, it should be noted that theincreased amount of pentane insolubles in Oils D and Adid not cause any difficulties with a reproducibility ofviscosity measurements and that can mean that the pen-tane insolubles were soluble in oil.

The next two analyses of oxidised oils were directedto compare the amount of oxygen incorporated into theoils. Acid number of oxidised oils is graphicallypresented in Fig. 8. Whereas acid numbers of fresh oilswere almost equal for all the oils, i.e. between 2.5 mgKOH g21 and 3.0 mg KOH g21, differences betweenacid numbers of oxidised oils were more significantranging from about 7 mg KOH g21 to 14 mg KOH g21.Similar to the amount of pentane insolubles, the highestacid numbers were also determined for the Oils D andA. There is probably a direct relation between theseparameters as more acidic oxidation products of oils caneasily condensate to form pentane insoluble compounds.

Almost the same tendency as for acid numbers wasobtained for an increase in carbonyl bonds concentration

Fig. 8. Acid number of oxidised oils.


Fig. 9. Increase in integral FTIR absorbance of carbonyl groups afteroxidation of oils.

in oxidised oils. Results are shown in Fig. 9. Determi-nation of the carbonyl intensity increase by FTIR spec-troscopy could reliably be done as the intensity of car-bonyl bands in fresh oils was low and did not exceed10 abs. units cm21. Intensities and spectral shape of car-bonyl bands in fresh and oxidised oil are compared inFig. 10. As well as in the previous measurements, themost oxidised oils were found to be the Oils A and D.It is interesting to note that the Oils C and F, which afterthe oxidation test kept excellent viscosity characteristicsand other properties, showed only a moderate oxidationstability expressed by an acid number and increase inintensity of CO band absorption.

4. Conclusions

A modified IP 48 standard test was used for testingthe oxidation stability of eight engine oils of the SAE15W-40 and minimum API SG/CD specifications. Theconditions of oxidation were chosen on purpose as more

Fig. 10. FTIR spectra of fresh and oxidised oils in the region ofcarbonyl absorption.

severe than would be expected in common use of oil inan engine. Therefore, the differences in oxidation stab-ility between the oils tested give no evidence for thequality of the oils within the oil drain interval. Rather,the results from the oxidation tests showed a potentialof the oils for prolongation of the oil drain interval,which is currently recommended by car producers to bebetween 10 and 15 thousand km.

Most of the eight oils tested were produced from sol-vent refined base oils. Only two of the oils were madefrom hydrocracked base oil, and base oil of another oilwas most likely made from a mixture of hydrocrackedand solvent refined oils.

None of the oils had superior properties in all charac-teristics tested. However, it can be concluded that theOils F and C were the oils with the highest oxidationstability among the SAE 15W-40 engine oils tested and,therefore, with a good potential for a prolongation of oildrain interval. The oil F was produced from a hyd-rocracked base oil.

Acknowledgements

The authors thank the refinery Koramo a.s. in Kolinfor financial support of the work. Many thanks also toDr I. Vaclavickova and Dr N. Vinklarkova for their helpand interest. Part of the work was supported by grantno. 104/00/0576 obtained from Grant Agency of theCzech Republic.

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composition and oxidation stability of sae 15w-40 engine oils

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