Well to start with, all gasoline in Cal is blended and each region is different and changes at the start and end of summer. As such, all the gasoline begins to turn to varnish at 6 months.... there is no way around it. The biggest issue with most of the fuel tank additives is the carb & fuel pump gaskets (of course the fuel lines as well...unless you have alcohol spec'd gaskets which have been available for 30 + years now). Marvel Mystery Oil is good to add a touch to the fuel system, run the engine for a moment or 2 so it gets into the carb....now things are lubricated. Although it is best to keep the fuel tank full (to eliminate moisture/air which promotes fuel degradation), I siphon the fuel out of mine about every 2 months and use in our daily drivers, then replace with fresh.
Fuel lines...if you haven't done it already, when they are due for replacement (5 years of life max), replace them with EFI fuel line (rubber hose) as this will last a lifetime!
Member: Never trust a person over 40 who drives a Chevy club
Flatheads ain't so bad!
Certified backyard mechanic I & II