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There is a study on about ten diff fluids that you can find with a google search. I'll note that Amsoil paid for it and came out on top although it was supposedly by an independent testing company. The top brands were relatively close, but Royal Purple was not one of them. It did not do very well. After reading it, I changed the RP 75W140 that I had just put in my E36M3 diff. There were also a few anecdotal reports on Bimmerforums of diffs breaking after running RP, but there were only a few, the diffs had all been in use for many years and there was no proof the RP was responsible for the failure -- just suspicion since there had been no issues with other fluids previously.
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