That government mandate on lighter weight oils in the 80'* was applied to millions of cars, resulting in a fuel savings on a national level. You won't notice any better or worse fuel usage in your car by changing oil viscosity.

If you want to go a little thicker, high mileage 10W-30'* are right smack in between 10W-30 and 10W-40 in thickness. If you're over 75,000 miles, you're suppose to be using HM 10W-30 anyway.

It takes HP to turn a oil pump,, thick oil like that is going to hurt efficiency, and MPG
GM has a great oiling system and a great oil pump(pressure) so thick oil is not needed
go with a good FULL synthetic oil like Mobil 1 or Amsoil 10w30 and change at
regular intervels 3-5K and beleve me you'll be just fine....$.02

Mac...I'd like to disagree about "suppose to be using" a high mileage oil over 75K. HM oils have not been out very long in the everyday oil world and are to me a gimmick to charge a little extra per quart.

Rant that can be skipped:
Every vehicle I've had has used the manufacturer recommended weight w/o a single issue. Currently my 97 Bville is around 149,740 miles and loves the 10w-30 Mobil 1 it gets. Doesn't use or burn any. Oil pressure is right on the mark with lower mileage cars.
End of skipable rant

No where in the owners manual does it say that a thicker oil or high mileage marked oil is needed or suggested in any car to my knowledge.

Bill,

It is mandatory that you use HM oil after 75,000 miles; it says so right on the bottle.

Call me a rebel. I'm with you Mac.

I could use 10w-40 in my 80 Buick no problem because it was designed for such.
With todays tighter tolerance engines and improved oil efficiency its not needed nor required.
I like to mix some 5w-30 with 10w-30 during the winter oil change interval but use straight 10w-30 all summer. If you have the engine oil cooler like Bill said you could run all day with 10w-30 no problem.

I just wanted to mention that I am currently using Valvoline MaxLife 10W-40 in my 3800. It seems to be doing fine, very smooth. Please don't get me wrong, I'm not advocating that everyone start using 10W-40, I'm just merely saying that no harm has become of it in my particular engine. I wouldn't suggest using it in a brand new factory engine with zero miles on it.

Your average 10W-30 is around 10.5 to 11.0 cSt @ 100C.

High mileage 10W-30 is around 12.0 cSt @ 100C.

MaxLife 10W-40 is 13.9 cSt @ 100C.

Havoline 10W-40 is 15.9 cSt @ 100C.

Certain oils run thicker or thinner with in their grade. So as you can see, MaxLife is a thin 10W-40, which is why I chose it.

Happen to know what Mobil 1full syn. 10w30 is????

i used too but forgot..........

10.0 cSt @100C. That is VERY THIN for a 10W-30.