Quote:
Originally Posted by Vigilante375
5w will do better in warmer environments (above ~30 degree's) compared to 0w. 0w will be better in colder environments (below ~30 degree's) than 5w.
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This is not always true, and I suspect is the assumption made by the OP's shop.
Partularly in the case of Mobil1 dexos oils for our Camaros, the 5W-30 actually has both a lower cold and lower hot viscosity than the 0W-40 Forumla ESP, as seen in the above links.
So the 5w30 is the better cold oil.