HelloSailor, you hit on exactly my question. Yes, it's a 1979 engine. And, while I downloaded the manual this month, it's original to the vintage. And, yes, oil has come a LONG way in 30 years. I'm tempted to go with Rotella-T 15-40 (universally available high quality oil). The 40 is a touch thicker, but I doubt anyone will ever notice. And the startup at 15 would be nothing but good.