Lead substitutes are for octane enhancement, not to replace the putative lubricity of the lead.
I've though for years (since my high school chemistry class in 1972, in fact) that the lead in fuel acting as a valve lubricant was pure bull-- no way it's going to stay METAL in a high temp oxidizing atmosphere, and metal oxides aren't exactly what I would call a lubricant (India stones are aluminum oxide, for instance.....).
Lower than necessary octane, on the other hand, will burn valves and pistons pretty fast! However, I'd believer current octane standards over what they were in the 70s -- things were pretty fast and loose in those days.
So long as the fuel you use is rated at what the engine requries, you are fine. Adding octane boosters above what is required can make for ignition problems -- it is entirely possible to make the fuel nearly impossible to ignite!
1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!