Brad Penn Oil has necessary Zinc protection for flat tappet engines, i.e. older engines like the slant 6. The stuff is pretty pricey, so I stick with regular 10w-30 and Lucas Oil Zinc additive to protect the engine.
Penn Grade 1
20W-50 #7119
Penn Grade 1
10W-30 #7150
Brad Penn PCMO
20W-50 SJ #7123*
Use the numbers (#) to find what oil works for you.
I've used just regular 10w-30 or 40, I'm going to try mobil 1 with the Lucas Oil Zinc additive.