Originally Posted by IMSA_Racing_Fan
If your Olds is a Rocket from 1978 and earlier it has .040 clearance on the rear main to provide lubrication to the rope seal. 15W40 was used for the five years I had her, not a bit of smoke or other issues.
Mine is 2 years older than that one. Nice car by the way. Any 77-90 b body can basically run forever and are so easy to fix.
We checked clearances the cheap way with plastigauge before installing it in the car and it was within factory spec. I don't remember spec so I'll take your word on that.
We also replaced the rope seal with a neoprene ford 292 seal which hasn't had an issue in the 13 years and 100k miles since then.
I'm sure it would run forever on 15w40, 20w50, or 5w30/10w30, I just prefer the quieter valve train I get with any of the 30 weights. If I was to race the car or tow through the mountains u would switch it out with a 40 weight or thicker, but in my usage these days, running it hard is hitting ~4500 RPM for a split second, not very hard on these engines.