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Just use a major brand of rear end lube, I use Valvoline 85W-140 High Performance, rated GL-4 and GL-5 in my Ruckstells.
This post on a blog about EP lubes gives so info, the cheaper non-brand or snake oil companies might have high acid sulfurs in the additives, but major brands likely don't.
Most major manufacturers have steered away from formulations with active sulfur EP additives to avoid staining yellow metals. In general, I would say the API GL-5 Multigear EP SAE 80w-90 lubricants marketed by Exxon-Mobil, Chevron, BP-Amoco, Castrol, Valvoline and Pennzoil would be safe for yellow metals. Also, well-known companies such as Redline, who specialize in motorcycle and sports car racing lubricants, would have products that don't stain yellow metals.
As far as the difference between GL-4 and GL-5, API GL-4 rated lubricants do not have the EP level to withstand shock loading. Unlike API engine service classifications, API gear lubricant classifications are not required to meet the specs of lower classes and it all depends on the application. In the case of the Triumph, it probably recommends a GL-5, but even if it only recommends a GL-4, I would go with the GL-5 fluid.
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