Stick w heavy conv. oil w that 3400 engine. My wifes old Pontiac GrandAm drank AMSOIL Signature Series 5w30 like water. I had best results from 10-30 & 10-40 conv. oil. I even ran several OCIs with Quaker State 10w40 Circa 1984-5 oil! It ran great. How do I know? UOA from Blackstone posted here a few yrs ago.