Can I use 5w20 in a 1991 ford ranger 2.9?

Might be a little light for a 1991. Maybe 10W30 would be better. Just IMO

I agree. Morning fordnut.

use that only in the winter- a COLD winter-

Usually stick with the manufacturer's recommended grade specified in your owners manual. In cold climates, this may be acceptable, just remember, as the oil gets dirty and old, the oil will assume the base weight, which is the 5 wt., so make sure you change oil regularly.

How many miles are on the truck? if it's over 150k I'd say you might want to start looking for a thicker oil. Using 10w40 in the summer and 10w30 in the winter something like that it may also depend on where you live too, I'm from Arizona and so what I do may or may not work better for your car