Brian,
There is yellow metal in most Triumph diffs (I'm not sure about TR7/8
offhand, but definitely TR2-6, Spit, Stag, etc), and I believe it can be
attacked (slowly) by using the wrong gear oil. The supposedly rebuilt diff
I bought for my Stag had fairly severe wear only to the brass thrust
washers, likely from the wrong oil; and I suspect that is also the problem
with my TR3A diff (although I haven't torn it down yet).
However, the problem is not whether the oil is rated GL5 or not; but rather
whether it uses an "active sulfur" additive. This additive is also found in
most conventional GL4 gear oils, just at a lower level than some GL5 gear
oils. The additive breaks down over time (combined with high pressure and
heat) to form sulfuric acid, which is what corrodes the copper alloys (aka
"yellow metal").
But the Valvoline full synthetic does not contain ANY of this additive, and
hence is safe for yellow metal.
It is my belief that this is true of any GL5 (or GL4) oil that also meets
the MT1 standard. Even though the MT1 standard is intended for heavy duty
manual transmissions; it includes a long-term high-temperature stability
test that the "active sulfur" additive cannot meet. But YMMV.
-- Randall
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