I rebuilt my 292 about five years ago. The breakin oil and first couple of changes was with regular 5w30 oil. Since then it has 5w30 sythetic.
There is never a good reason for using heavier oil than is needed, especially in a cold climate like Canada.
Yesterday the temperatures in Southern Calif was over 110 deg in my area. I took the '67 out at mid-day for a couple of errands making four stops.
The AC was blowing cold, the engine temp gauge was in the cold and the oil pressure gauge was above the half where it always is. The '67 GMC gauges has no numbers, just C & H and L & H.
I would love to use an oil with more zinc, but there is no way I'm going to use w40 oil to get it.
