If you're planning a rebuild down the road, forget about the lead substitute. If you've been running unleaded through it and it's not giving you trouble, same story. If, on the other hand, you want to keep the existing valvetrain as long as possible, get a bottle of lead substitute. Better safe than sorry.
My preference is 92 octane with a squirt of lead substitute. Zenith-topped 248 runs like new, and I'm not changing the formula until it acts up.
