Because it is like the last oil grade you want to use if it is conventional. Back in the 80's they thought is was the cats meow but they found out it is not and no new cars from Detriot are approved for its usage and have nor been for years. It has to do with the hi VI content of oil the can cook out and gum up a modern hot running engine. (grant that some 10w40 are better than other quality wise but it is best avoided) Use 10w30 spring, summer, fall and maybe 5w20 in winter if it get below zero a lot.
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