My personal experience of low winter use over the past 40 years is this:
The only time I had fuel go off was when it was a low level in the tank, and it turned greeny yellow and covered the carbs in a crusty deposit.
With a full tank over winter I never had a problem.
When I replaced the tank on my MGB, there was serious scaly rust along the front edge, the lowest point inside, and around the recess where the filler went. The rest of the inside of the tank was shiny steel.
In terms of 99 octane and ultimate fuels, I just use them, because I believe the detergents will keep things cleaner. Maybe it's a con, but there are more important things to worry about!
