Few people realize that the gas in the early 70's contained 3 full grams of lead per gallon of gas and regular was 93 octane. Like the octane rating, this amount of lead was lowered over the years little by little so the public would not notice it as a dramatic change to the car's performance or longevity. Cars that required it most would go bad first and others would take a little longer. This way, consumers would not realize it was the gas that shortened the life of their engine. The idea was to get all of those old cars off the road and replace them with less polluting cars. A few tenths of a gram of lead was reduced every 6 months until now and it's all gone. You probably have not realized that you have been running unleaded fuel for a long time now.
As far as additives or octane boosters, we have nothing that we can recommend at this time. Lead requires a sealed environment to be dispensed. I do not believe anyone can legally sell the Tetra Ethyl Lead to the consumer for an additive. Some suppliers are claiming to have a substitute lead material, which I have not found to do anything in my testing. However, it seems to leave residue or deposits quickly on the plugs that were never a problem before. Octane boosters are mostly made from the solvent families and do not actually increase the octane of the fuel, but "SLOW" down the burn travel so it flashes slower.
In the air cooled engine this simply makes the engine run rich and hotter. I have yet to find one that actually had anything in it that has an octane rating higher than or even as high as the pump gas. Whenever I questioned the suppliers of these additives, "What is the actual octane of this additive?" They did not know and simply ignored answering or hung up on me.
Even many so called "RACE FUELS" are actually pump gas with retardants added to slow the burn travel so it will not ping. I, personally, do not use such fuels as any I have tried simply did not work in the VW. Once I found the VP Racing Fuel, I have used or recommended it or its equivalent such as Union 76 race fuels like NASCAR uses, Cam 2 (Sunoco), Phillips or Sports race fuels or any fuel that is manufactured with the proper ingredients. I find that when a new fuel comes out and I tell them I am buying it to go directly to the lab for testing to see if in fact it is a true race fuel, the pretenders do not want to sell me their fuel.