I need to know what type of Oil I am supposed to be using in my truck

I need to know what type of Oil I am supposed to be using in my truck I have been looking all over the internet/on auto stores websites and still cant figure out exactly what type of oil I am to be using in this truck. It is a 1988 Chevy Scottsdale C1500 2 WD. Please help me figure this out. Thank You!