In other words, what engine specs determine the oil weight?
Does it depend on the valvetrain? Does it depend on the crank bearings? I have a 400 I want to rebuild. Still in the planning stages. The label on the oil cap won't matter any more, obviously. How is this determined properly?

Thanks!

 

Oil weight is determined by the temperature range the vehicle will be used in(summer only vs summer & winter), bearing clearances, and the intended use. Since I rebuilt my 350, I have been running standard 5w-30 non-synthetic oil with a ZDDP additive. I used the same stuff for the initial break-in as well. I haven't seen any signs of problems during oil changes. I only drive it during the spring/summer/fall and garage it over the winter. I wouldn't go much more than 10w-40 on any engine though. Especially if you're driving it in cold winters. I run that in my truck engine due to slightly lower oil pressures and it can take a little while for the oil pressure gauge to come up on the coldest winter mornings. I have another engine ready to be assembled for that truck so I'm not too worried about ruining the one that's in it.