Gear Oil with API GL-5

What the high additive content is for

In a hypoid axle the pinion and the crown wheel are offset. As a result the tooth flanks slide against each other far more instead of simply rolling, and the contact pressure reaches values at which an ordinary lubricating film breaks down. The sulphur-based EP additives in a GL-5 form a reaction layer exactly there, preventing the flanks from welding.

The same additive content is the reason GL-5 is out of place in a synchromesh gearbox: what protects steel attacks brass and bronze.

LS: only with a limited-slip differential

A differential with a plate-type limited slip needs an oil with a limited-slip additive. Without it the diff judders and growls when manoeuvring on full lock, and the plates wear. In an open differential, by contrast, an LS oil does no harm.

Some manufacturers specify the additive as a separate product, others require an oil that already contains it. Which of the two applies is stated in the maintenance schedule.

Common viscosities

75W-90 is the most widespread grade for car axles; 80W-90 and 85W-90 are found on older vehicles and in the commercial vehicle world. With towing and sustained high loads it is worth a look at the axle manufacturer's approval list, on commercial vehicles usually ZF TE-ML.