Many years ago, about when they were changing over to nolead gas, I was sitting on a plane next to a PHD chemist from a major oil products research company who was quite familiar with the issue. (They were developing the processes to make the unleaded gasoline.) We first started talking because I had worked one college summer job with the company.

He believed that with all the miles that I had on the car being run on leaded gas, that there should be sufficient lead that would have permeated the seats to protect them.

That said, I have still used either a lead substitute or real tetraethyl lead with every unleaded tank. It's just my cautious, conservative and redundancy-seeking nature, I suppose. (I need the octane in the 400HO anyway to reduce pinging so the real lead also takes care of that.)

I just use lead substitute in my '68 ford with a small V8.