Is it OK to use 5w-20 or 0w-20 oil in a 1991 Firebird?

[Andrew Allison Adams]

It's NOT a Trans Am and NOT a Formula
It has the L03 5.0L V8

The book recommends 5w-30 for all temps
10w-30 is ok above 0
straight SAE-30 is ok for 40+

but I also know that the 20's can get better gas mileage and is used on may newer cars. Ford even retroactively recommended 5w-20 back to 1991 on most of their cars with only a few exceptions (v8 SHO is the only one that I can recall now). There is also a controversy on whether or not the 20 is enough to protect the car.

This is why I'm unsure though

help me out if you can

 

[mike g]

5-20 for winter 10-40 for summer

 

[Goatman]

It all would depend on the mileage of your engine if your car has over 75xxx miles I would NOT recomend using 5w30 even in winter it does Not provide enough protection (viscosity) under normal conditions when the car has higher miles ..
the 5w20 or 0w20 are even thinner than the 5w30 and will NOT give the engine enough protection bearings and seals will be ruined by the thin oil ..
if you live in a warmer climate you can use 10w30 year round if temps get below 0* then use the 5w30 in the colder months but under No circumstances use anything thinner ..
even under the best conditions you would NOT see enough of an increase in mileage (fuel) to make any difference and the wear on the engine would Not be worth it ..