Tony,
Whereas EP oils are essential for hypoid gears, they also have an important duty in other situations where the gears are in contact under extreme pressure - hence the oil's description.
There is a contrast between the steady state rolling tooth contact within a gearbox in an industrial application and the complex dynamic loading that goes on in a rear axle. Any axial movement of the pinion plus flexing within the carrier bearings will produce small amounts of shear; but the big unknown, in terms of pressure, is the effect of impact. Even in a correctly set up axle, vibration through the driving train "finishes up" at the axle (the wheels are big dampers!) where it can add an impact load to the driven load. Wear, poor assembly, nearly running out of oil and a harsh clutch add considerably to the need for something better than a straight oil - and then there's the way we treat our cars/specials.
I think these are some of the underlying reasons why Austins specified Hypress EP 140, or equivalent. This is what I shall continue to use, and I shan't lose any sleep over attacks on yellow metal.
Ron

Location: Thornbury South Glos

Well, it may be boring but, I tend to follow oil company's advice. If they say that they DON'T recommend one of their products for a particular application that carries more weight with me than when they DO recommend a particular product.

There may well be people, somewhere in the world, who have used nothing but margarine in the diff. for the last 60 years without ill effects but, I think I'll just keep on following the supposed experts in the industry's advice. After all, why would they lie?? They end up selling me one of their products anyway and the price difference is insignificant.

Ian Mc.

Location: Shropshire

That would smell a whole lot better than normal back axle oil and, if I broke down in a remote part of the world, I could live for several days on it!

New advertising campaign emerging: "I can't believe it's not back axle oil".

I think the snag with margarine,Ruairidh, at least in the western world, it hasn't been available for human consumption for some years.
When butter was in very short supply in WW2, margarine was produced as a healthier (and more available) option. Today, however, it has been seen fit to ban it!! Cela vie.

Ian Mc.

Location: Shropshire

Ian,

In the Principality of Glasgow we eat many things the rest of the world sees unfit for human consumption...

Ron,

At the risk of boring everyone - Industrial applications notwithstanding- the GL4 EP sulpher phosphorous additives give minimal advantage in spiral bevels where there is no continuous sliding contact under pressure, just, as you, indicate shock loads which need high viscosity - hence SAE 140.

Obviously you can of course do as you say.

Before someone else says - I will stop pontificating and shut up.

Tony.

Location: Malvern, Melbourne, Australia.