What kind of oil should be used for a 1989 Cadillac Deville?

i have a 86 with a 305 and OEM is 5w30 but if higher milage i would recommend heavyer I'll run 20w50 or straight 40 in summer with no problems but that's me also use 40 in winter once in a while hard to crank but after a couple of cranks it fires and have had no problems ut once again that's me