Although only one grade of leaded avgas (100LL, standing for 100-octane, Low-Lead)1 is in common use today, there were formerly a number of different leaded avgasses with higher octane ratings and tetraethyllead (TEL) contents (things like 100/130 and 115/145), catering to the large, high-compression piston engines of decades past; these higher-performance blends are now produced only in small-volume special-order batches for applications where even 100LL doesn’t cut it, such as air races.
Are there differences between the various grades of leaded avgas other than their lead content, or could one convert 100LL into (say) 115/145 simply by adding more TEL?
1: 100LL is only “low-lead” when you compare it to the even-leadier avgas blends of days mostly past; it still has a considerably-higher lead content than leaded mogas ever did.