Lubrication 23 - Gearbox Oil
It is not so much that gearbox oil is much less demanding, yet other requirements are important in the case of gearbox oil. One must first of all, make a distinction between lubricating oils for manual transmission and oils for fully automatic transmission. Because oils for manual transmission are also used for hypoid drives, they need a high shearing strength. At the same time the lubrication ability must be limited, so that between the synchroniser ring and/or the gear wheel cone enough friction can be built up. The life span is also much higher than with engine oils, probably due to lower temperatures.
How it works
According to the API norm there are two important performance classes for gearbox oils. 'GL4' is suitable for all older designs of gear-boxes, also those with hypoid drives which was first introduced in the period shortly before and then just after the Second World War in motor car construction. Oils with the specification 'GL5' are suited for more modern vehicles, also those with offset. These are also available as synthetic and multi-grade oils.
GL5-oil includes up to 6.5% additives (GL4 up to 4%) for protection against wear. It is of course also suitable for self-locking differentials with restricted slip. 'GL6' which provides even more wear protection, is intended only for special uses: particularly high shearing strength at high speed and a stronger offset.
The SAE classes
Like all other oil, gearbox oil contains of course additives against oxidation and for thermal stability, but also against copper corrosion and enhancing other component protection. Of course none of the contained additives may be aggressive towards metals used in the gearbox. All in all, there hardly is a choice with passenger car gearboxes. Often there is a lifetime filling and/or the oil is stipulated. Using oil of a different specification is considered occasionally to reduce mechanical noise or to achieve a lighter engine turnover, this measure however, is not without problems and seldom effective. 06/08