>From several sources (Skinned Knuckles, Flying Lady, etc):
EP (= Extreme Pressure) gear oils are compounded to maintain an oil
film under the high pressure encountered in a hypoid differential,
which combines the worst conditions for gears -- axial and lateral thrust as
well as sliding (ah, the price we pay for a lower floor). These oils
contain sulfur compounds, which is why they are so tenaciously
aromatic. These compounds _will_ attack most yellow metal alloys,
including brass, bronze and copper. This is clearly a problem for
boxes with synchro rings or bronze gearing (such as found in the power
brake drive of a Phantom).
Bottom line: Use what the mfr recommends. You can get heavy (90+
weight) lube without EP additives if you have to have syrup in your
box. There are a few vendors that even supply 600W (non-EP) lube,
which was commonly used in old Fords (T's and A's), which is what I
use in the steering box of my Roller.
A. B. Bonds
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