5w40?

[BigRed2]

Can I run 5w40 oil all year,even in summer or should I stick with 15w40, I have a 1999 powerstroke 7.3, I'm in Michigan it only gets at the hottest like high 90s-thanx John

[Dave]

Sure can. The 5w and 15w portions are only the cold flow rates. All that means is the 5w will flow better when its cold.

They both lubricate exactly the same when the engine is at operating temp