There have been changes in the formulation of gas, but they are for the worse, not better. Older engines needed the lead for the soft valve seats, but they also benefit greatly having the lead in the bearings. Any old engine that ran its first few years on high content leaded gas stores much of that lead in the bearings for many years afterward.
A few years ago, I pulled the 350 /300 out of my Son's 69 El Camino for a new rear main seal. This engine ran its early years on leaded gas, and then once the lead was gone, it lived completely on unleaded for many years.
On opening the rear main bearing, we found a lot of nice slippery grey lead material filling every crevice. You could wipe piles of soft lead off on your finger. We didn't remove that lead, we simply replaced the seal, and put it back together with the stored lead in there to protect the bearings.
An older engine that hasn't been torn apart to rebuild stored that lead almost forever. Leave it in there. It is very good for it.
Lead substitutes do not leave a long term lubricating residue in the engine like lead does. Instead of using substitutes, go for an occasional fill of racing gas with real tetraethyl lead.