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Has anyone used Redline MTL in winter? (real winter, not CA/FL winter)
MTL is probably a bit thick for sub-zero northern winter temps. That's not to say it's 'bad,' but yeah it'll be tough to shift through the thick stuff until things warm up a bit. My guess is that the thinner Syncromax (or similar) would be better in that type of climate. If you're worried about the longevity of the RP Syncromax, you could consider a 50/50(ish) 'cocktail' or RL MTL plus RL D4 ATF for a resulting thinner fluid.
 

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Good advice here. Back on page 1? I think of this thread, LCSMAN (a respected member when he was around) described the Royal Purple very well - it quick shears/loses protection properties after a few thousand miles. I can attest to that, as I ran syncromesh or whatever they call it, for a few of the many trans oil changes I've done to my M5 420g over the years. I've personally put about 180,000 miles of the 280,000 -ish miles on this transmission. The Royal Purple starts out amazing, but after a while it goes to crap and is like water when draining it. Before the Royal Purple and ever since then, I've just run 80/90 weight synthetic gear oil. Usually Mobil 1, but the latest fill was Valvoline 80/90. I change the fluid roughly every 50k miles, onvce or twice I changed it more frequently because the trans was out for a clutch or I was about to drive around the USA.

As a related side note, I ran Royal Purple gear oil in the differential for a stint. When it came time to change, it was still purple and looked pretty good. For cost/availability reasons, I've pretty much stuck with Valvoline synthetic 75/140 limited slip. No issues pounding the miles on the car with that oil either.

Final note - I live in the northeastern part of the USA. We see subzero temps in the winter sometimes and up to 100F on rare occasions in the summer.
Hmm, that's odd about the Valvoline Limited slip oil. I contacted the Valvoline dealer here in Norway and they told me they didn't have that type of oil. So for me I have to use the Castrol 75-140 limited slip oil.
 
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