340 4 barrel hasn't ran in 15 years need help.

The any fuel left in the tank is guaranteed to be absolute crap, and it can badly mess up the valves and maybe a piston by making them stick. Don't even think of using any of it, not even of the idea that you can water it down with fresh fuel. Drop the tank and get it all out, and clean out the tank of any trash or old fuel residue. Blow out the lines and clean out the carb.

Oiling the cylinders is good; I start with WD40 or Kroil and then rotate the engine some, and then add a bit of light motor oil.