Not available legally and not very smart to use -- lead oxide is an abrasive and will grind up your engine's inwards (the main reason cars used to last only 100,000 miles or less), it is big time toxic, absorbed through the skin, causes irreversable brain development problems/mental retardation in kids, and isn't necessary in modern fuel.
Use Premium 92/93 octane, you don't need more.
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!