bennie, I own a mid-80's Fiero and right on the oil fill cap if embossed the
directions to use 5w-30 oil. That means for 20 years at least the manufacturer
GM has recommended multi-weight oil.

in fact, GM has recommended multi longer than that, since the 1970's

go ahead. tell us it was a GM plot to sell us more cars.

manufacturers dont make recommendations for no reason, follow them and
you will never have any trouble.
if you are having trouble finding a 30w oil any volvo dealer will sell
it as a lot of volvo equipment uses that grade.