older cars tend to have some (more or less) deposits from gas and combustion produtcs. this stuff usually looks like brown, dense goo on the injector nozzles, and valves. cumbustion products usally are black carbonized hard crap on insides of valves, and exposed parts of the combustion chambers.
this can cause rough idle, mainly cos injectors cannot atomize gas properly. techron is made of aromatics, plus some nonionic detergents and it cleans all this crap off. made by chevron, recommended by bmw (or used to be).
isopropanol or methanol:
just regular alcohols. the intent here is to remove all water from gas, which happens always. ambient moisture is in the gas tank, and it condenses on the sides, as temperature goes down. water does not mix with gas at all, and occasionally can interfere with smooth flow thru injectors at low delivery volumes.
both alcohols are very hygroscopic, so they mix with water in any proportion. however, isopropanol is better at mixing with gas, whcih is these days mostly aliphatic hydrocarbons. so like it over methanol for that reason.
you can get all of this at autozone or any equivalent car parts store. techron about $5 per bottle, either alcohol about $2. one bottle of each per tankful, and you are done!