Carl - I use only 100LL (not because I want to) and the Decalin is used to avoid or at least minimize lead deposit build up. Lead build up will do the same to engines as carbon will but (in my experience) more frequently and in more places. Decalin causes the lead to end up in a different compound during or after the fuel is combusted and it flows out with the exhaust rather than building up. I've not come across it being used as any kind of cleaner - can't talk to that - but my tanks have not (yet?) shown any deleterious effects.