I know it is comparing apples to kumquats, but the factory specs for the GMC six cylinder engines call for a minimum of:
Oil recommended for use between 32 and 110 degrees F was SAE 20
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Wow, are you sure that isn't a misprint? Most always it was SAE30 in the day?
When it comes to oil and viscosity, I try to relate to the important and costly engines. Most all large piston aircraft engines in the day used SAE 60.
Most all large marine as well as locomotive engines use SAE 40, and even Detroit diesel 2 strokes only recommended SAE 40. Reason viscosity takes the weight, maintains oil film.
None of the posted engines ever use multi viscosity oils.
