TheSearcherMan said:
It's my understanding that the additive that eats the "yellow" metal, is sulfur.
Well, more accurately, it's a compound that contains sulfur.<div class="ubbcode-block"><div class="ubbcode-header">Quote:] The additional sulfur is what makes it possible to meet the GL5 specs.[/QUOTE]True, but there are other additives that perform the same function without containing sulfur. Plus synthetic oil, with it's higher base film strength, needs less additive help anyway (to meet GL4/5).<div class="ubbcode-block"><div class="ubbcode-header">Quote:]I also question how many years it would take to do any damage, no one has answered this question to the best of my knowledge. [/QUOTE]I believe that's because the time required is highly variable. The additive does not do the damage directly; instead when the additive is called into play (by extreme pressure and heat) it breaks down into other compounds, including an acid. It's the acid byproduct that does the damage. So, how quickly the damage occurs depends on how hard (and how often, etc.) you drive your Triumph.
I was dubious myself, but I recently rebuilt a Triumph diff that appeared to have had conventional GL5 run in it. The "old gear oil" smell was very strong, the brass washers were badly worn, and the blackened material (not sure offhand what compound it is) was very much in evidence.
And since loosey-goosey differentials (indicative of worn brass washers) seem to be quite common in Triumphs, perhaps the problem is more prevalent than we realize.
But just to reiterate, as I see it, the big problem is that "GL-4" does not guarantee the oil is safe for yellow metal, any more than "GL-5" guarantees it is not safe. Both specs have tests for "yellow metal" corrosion, but the GL-5 test is actually MORE stringent than the GL-4 test! In fact, most GL-5 oils will pass all the GL-4 tests (and there used to be "combination" oils marketed).
So, MT-1 is the spec to look for, to be sure the gear oil you are buying is compatible with "yellow metal"; plus a GL-4 or GL-5 rating to be sure it protects the gears in the diff.