Cold flow with 10w40 is not a whole lot behind a 10w30. After all, both are 10wXX oils. I doubt that 10w40 is going to ruin an engine, but I would not run 10w40 if the oil pressure is in spec with 10w30. I have been runing 10w40 in my 4.9L F150 and with it, oil pressure is in the lower range of spec or about 44 psi.

 

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TP, the 10w40 will pump slower when cold, no matter how you slice it. It is also part of the reason why no 10w40 meet a "Energy Conserving" rating. There is just too much internal restriction with this heavier oil.

Sure, a 10w40 oil, you can use it. Engine will still start and run with it. But why? In a modern, FI engine, a 10w30 oil will provide better performance. Better fuel mileage. Any engine can idle and run with much less restriction with a 10w30, while providing excellent heat and sludge protection.

Basically, why not use the overall better oil, a 10w30? Use what you want, but 10w40s and above are practically obselete from anything (engines) built past 1980.
Ed
PS 10w40s won't destroy or ruin an engine. They are not the best choice, so why not use what is best or very close to it, for an engine?

 

Well Ed, after reading your message I sold off my 5 quart stash of 20w50 realizing I don't need it to thicken up the 10w40 for my F150. It's a start anyway.

 

Ed,

Sorry to differ, but SAE J300 specs for oil viscosity state that 10W has a certain cranking and pumping viscosity at certain temperatures, regardless of the second number, 30, 40 or 50. There are no XW-40's that are "energy conserving" because the basis for that designation does not allow it. Higher viscosity at 212F compared to XW-30 will cause some small amount of greater internal "drag". HTHS viscosity minimums for 10W-40 is actually the same as 10W-30 in J300, 15W-40 is higher.

Jim

 

I use 10w30 in both of my trucks and have had no problems.