Downside, like switching to synthetic oil, is if your car's not new(er) then you may spring leaks becasue of the improved flow and different characteristics of synthetic. However, I am confident that synthetic fluids decrease wear and flow dramatically better at low temps. I run Red Line Syn fluid in both of my trannys, and was lucky enough not to spring any leaks. It's all I run.

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Best regards, Michael
'92 500E
'88 300TE