Oil for a Slant Six that's been sitting a long time

Since it's going to be a while before you try to put the engine underfire, IMO, it is going to be more effective not to change until you're ready to light it up. I'd recommend a little #2 diesel or Marvel Mystery Oil in the top of each cylinder, let set a day, and turn over engine by hand.

When you light it up, get it up to operating temperature. Shut off and drain with the oil hot. Refill with engine with a medium quality oil less a pint. Substitute the pint with AFT. Put a 200 miles on this soup and then change while hot. This time make it a regular change, good quality oil, a Zinc additive (if not already in the oil), and a decent quality filter.

The reason for the ATF is to take advantage of its strong detergent qualities. Changing the oil while it is hot helps get rid of junk that will be in suspension in the oil when hot but would congeal and settle when cold.