"Hypoid" is not really a question of oil, so much as a question of
gearcutting. Old (1920's) rear axles used straight bevel gears to form
the crownwheel and pinion. These had two disadvantage, the pinion
shaft meets the crownwheel on its central axis, and the straight cut
gears are noisy. By using a more complex "hypoid" gear tooth shape (if
you look at a pinion, the teeth appear twisted) these problems can be
addressed. The more gradual engagement of the teeth along their length
reduces noise. By careful design of the geometry the pinion can be
made to mesh _below_ the axis of the crownwheel. As the centre height
of the crownwheel is fixed by the wheel height, this allows the
propshaft to be lowered relative to the car body, giving a clearer
floorpan and lower centre of gravity for better cornering. Hypoid
bevels are now universal in this application.
Because of the sliding contact that hypoid gears make, their
hydrodynamic contact pressure is higher. To be suitable for use with
hypoid gears, a lubricant must be capable of resisting high pressures.
Oils with "EP" ratings (Extreme Pressure) such as EP90 are required.
Some brands describe themselves as "hypoid" instead, a term which is
synonymous with EP. GL-5 is a formal API standard for this type of oil
(comparable to MIL-L-2105B/C/D)
So thats that resolved then - you can sleep easily now ! Thought I had better preserve this bit of the net 'cos is came from "74551.2327@CompuServe.COM" - somehow I doubt that email address still exists !