Which oil is best for a Olds 98 in warm climate
Which weight should one use and which advice on length of time to keep break in oil in the crankcase?

 

Depending on bearing clearances, probably 10-40 weight oil. If you broke it in with moly camshaft lube on some parts, it should have been changed after the 20-30 minute cam break in.

 

If you are asking about break-in oil, straight SAE 30 is typically used. I'd be more worried about using a high quality break-in oil for flat tappet cams. I usually change after cam break-in then again at about 500 miles.