My father has always had this rule. I know it could sound stupid but it's never gone wrong with us. 4 Bangers get 10-30. V's get 10/15 - 40.. Anything higher is just to keep an old engine from leaking until the motor goes. And I point to my father's old 85 Ranger.. We put 20 - 50 in it and it quit using oil until it finally threw the rod. High Mileage, the previous owner's negligence during the startup cycle and Highway driving in such a low geared vehicle contributed to it's demise.
My father also HATES synthetic oil. No real explanation other than he thinks it kills engines. Although while I was still receiving Hot Rod, my wife found the alluring Royal Purple.. She wants it in her car. I'm happy to oblige