Since Denver gets cold in the winter,(at least it used to) and the high mountain valleys get *really* cold, I have been considering taking advantage of the greater viscosity spread offered by synthetic gear oils for my 1986 300SDL. There is a product, Valvoline, I think, that offers a full synthetic GL-5 in the 75W-90 weight. I like the cold weather lubricity offered by the 75W and the hot weather protection offered by the -90. Any thoughts? Are there differential seal issues with synthetics as there are with engine seals? Frankly, the seal issue is probably my greatest concern.

Thanks for all replies.

I use Red Line 75W-90 synthetic in both of my cars, and have had no leaking probs. The wagon has 153k, the 500E 52k and neither was upgraded to synthetic until they were at least 6 years old.

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

I think you will be fine here. No pressure lubricated parts. Be sure it is thermally stable for limited slip diffs.

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