First of all unleaded gas is a lot more volatile than leaded. When unleaded was mandated in 1975, we had to put in submerged pumps because our suction dispensers would not pump the unleaded gas. Leaded finally became totally unavailable around 1990 and the few suction dispensers left are used for diesel. Alcohol makes it even more volatile. I haven't heard any additive that helps overcome this problem. I have 7 GMC's, some of which had Rochester carbs that were put on by previous owners. I have replaced all of them with Zenith carbs, which of course, originally came on the GMCs. Performance is noticeably better. I especially like the ones that were known as a service replacement carb. They had an adjustable needle valve in the main metering jet. They are getting really hard to find now.
As for the lead additive, it is not noticeable on on engines that do not see heavy service. Back when there were still a lot of 455 olds engines in boats and motor homes, it made a big difference in valve seat life. Most of these engines are now out of service or had the seats replaced with hard seats. As for the tax incentive on alcohol, it is for the producer of the alcohol as an incentive to build more alcohol plants. There is an 18.4 cent Federal gas/Lust tax on gasoline sold in any state.
