can i use diesel oil?

All the current popular HDEO's (*h*eavy *d*uty *e*ngine *o*il, colloquially referred to as "diesel oil") have corresponding gas engine certifications (SM, SJ, whatever). Just check the API certification label (starburst) on the bottle/jug. I don't know why anybody would not use them today in old muscle-era cars. Today's car oils are designed for today's car engines to flow better in small, high-revving engines with average lower viscosity aimed at better fuel mileage ratings.

I've run nothing but HDEO (5w/40) in my Mopars for going on ten years now. Good stuff.