Automotive Gear Oil #OilChat 62

Automotive gear lubrication is a very interesting but often rather controversial subject. We have discussed the topic in previous editions of our newsletter but there still appear to be grey areas surrounding the issue. We have therefore deemed it fit to revisit the subject with specific focus on API GL-6 gear oil.

Lubricants for automotive gear applications are classified by the American Petroleum Institute (API) by means of GL ratings. For easy reference we have summarised the API classifications as follows:

API Categories GL-1, GL-2, GL-3 and GL-6 have been declared inactive even though oils may still be marketed with these designations. Performance tests associated with these categories can no longer be run because parts or test installations are not available anymore – the API GL-6 test protocol in particular.

Raison d’etre for API GL-6 surfaced in the mid 1960’s when Ford needed better protection in their pickup trucks and GM developed the revolutionary front wheel drive Oldsmobile Toronado.

The V8 powered Toronado had a differential with a very high angle of gear contact for power transmission to the front wheels.

The abnormal angle of contact was the result of a differential with a very high pinion offset (discussed in OilChat No. 14) hence a higher gear oil category had to be developed. This category was later defined as AP GL-6 and the Toronado differential was actually used in the API GL-6 test procedure.

Interesting to note is that in addition to the higher pinion offset, the 1966 and 1967 model Toronados had sun gears instead of spider gears between the axle shafts. The power packed V8 Toronados also suffered from high temperatures in the engine compartment and very high loads and pressures in the drive train.

API GL-6 level of protection is still claimed by some oil manufacturers but can no longer be evaluated since Oldsmobile stopped producing the Toronado differentials many years ago. A shift to more modest pinion offsets and the obsolescence of API GL-6 test equipment have greatly reduced the commercial use of API GL-6 gear lubricants. Nevertheless, some manufacturers of high-performance cars still specify this level of extreme-pressure performance for their vehicles.

If you have any questions regarding gear oil or any other lubricant related issues, simply mail us at Our experts are at your disposal and ready to provide you with advice and guidance