I have to use Rotella 10w-30 in my old stuff all flat tappet cams . I didnt always use it and have had 2 cams fail lobes went flat and i bought a 1982 Chevy that had a bad cam. I dont think there are very many oils with the zinc that can be used in a flat tappet engine. I guess i am behind the times thinking there are alot of these old things steal being used.