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My dealer puts in FM booster diff oil regardless of noise. I had it in my diff after my 1200 mile service, again after a seal was replaced, and again after I got a replacement diff for a strange clunk that was not normal. Never did I complain about a grind, but I still got the FM booster fluid.
I asked my dealer for bottles of diff oil to change my own fluid last year. They gave me three bottles of 75w-140 diff fluid with FM booster. I know castrol saf-xj (now known as castrol syntrax limited slip) is the older fluid or what comes in our cars when brand new. So I asked for it and the parts guy said FM booster is only fluid we use on M differentials. I guess its the new standard fluid. But thats just from my experience.
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