Lead was a bad idea in the first place. (But we shall ignore the health concerns for now..)
It's value as a stem lubricant is highly over-rated in light of chrome or nitrided coatings, and it's digestive effect on oil makes that a bad tradeoff.
The basic benefits of tetraethyl lead were two things: octane increase, which is a fart in the dark on a flatty, and insulation against spot-welding of the valve to the seat. That IS a concern, particularly if you run a stainless valve against a castiron seat.
So the ironic delemma that we are left with is that modern manufacturers of valves put out even nitrided units with a stainless core. Duh.
This means that they expect everyone to have hardened (not matrix) seats.
It's a no win situation at that point. Unless you have the wallet to arbitrarily replace your flattie's valve seats with hardened ones that can easily shrink at the slightest sign of an intake leak, advanced timing from points cam wear, etcetera..
This whole scenario leaves us with either the choice of going back to the most primitive of materials, (because it works!), or spending incredible amounts of cubic dollars on the latest fad of hightech coatings.
So in our despair,... we turn to snake oils.