I use to run 20w50 in old high mileage engines. Then I backed down to 10w40 .

Had a smokey old 307 in a 69 C10 that I poured anything in. Never blowed up, just used alot of oil. Gave it away when I found that 402.

I use 10w30 + STP in old motors now and it does fine.

Newer stuff gets what the book says.

1962 C10 with a 235 6cyl -- all of the drive train seems to be original.
Some of this story is in the Side Lot
Some people like a new truck. I liked the old ones.