[LakeCleElum]

20-50 Brad Penn or Valvoline VR 20-50. No synethic Zinc is very important.
I have a 73.5 and had the same mag case engine for many years with the same usage until I went with a high compression 3.2.

Your engine can have oil pressure issues when/if the temp gets too warm. If you see oil temps in the 230 area, your oil pressure get will drop into the danger zone. If that happens, bet back to us for advice about cleaning the engine mounted cooler and maybe adding an exterior cooler that the later cars came from the factory with. Before trying to save a few bucks on cheap oil and no cooler, go price a full rebuild on a mag case engine. $18,000 is the current price range.
Your 1st post, welcome. Now, pictures are required of the great 73.5 model you have.

G'luck...