Lord, fuel cleaners are going to be the next oil threads! :tsk: Anyway, I've almost as much work with cleaners as I have with oil and here is how I break it down:
High maintenance (ie; every tankful)- Redline SI-1, Lube Control FP+, RLI BioPlus.
Casual Maintenance (ie; couple times a year)- Techron (or BMW stuff), Regane, Lucas, Gumout etc.
Low Maintenance (ie; aggressive one time use)- BG44K, Berryman Chemtool, Seafoam (or Deep Creep) Ventil Sauber.
Obviously the high maintenance ones are the mildest. They are slow cleaners and will clean over time/miles. They are good maintenance doses to keep an already clean system clean and provide some fuel lubricity. They also have the least effect on the oil when used in proper dosage. Basically wait til the low fuel light comes on, add proper dose, and fill tank the remainder til full. It does not hurt the fuel system to get to the warning light, there is plenty of fuel left to pump, cool, and lube the fuel pump. Our tanks are saddle style and have pick-ups on both sides to draw fuel evenly. There is a function in the OBC that will show how much fuel is left in either side. Look in the "easter eggs" thread to see how to do it and have a little fun.
The casual ones are a little more aggressive and should be used sparingly. Best used in conjuction with an oil change (use them BEFORE changing the oil) to keep the cleaning residuals out of the oil. Not good to have a fresh oil change get contaminated and keep it in the sump for the duration. IMHO, the Techron/ BMW stuff has shown better results. Plus it's easy to find.
The low maintenance stuff is very aggressive. They are usually the dump/ 20 minutes idle time varieties. The oil MUST be changed afterwards or you run the risk of having these strong solvent compounds literally eat the softer metals out of you engine internals. Their favorite snack is bearing babbit since its primarily lead, copper, or tin alloys. Also a good idea to change out the fuel filter too since it will most likely contain a good bit of contamination. These are best to use on higher milage cars that have had "neglected" fuel systems. Get them clean and then switch to a high or casual maintenence cleaner. Usually one or two treatments does the trick.
Modern fuels are really complex these days, not only is there just ethanol to worry about. There are drying agents and competitive aromatics to boost/reduce fuel vapor pressures, and high tech detergent additives like amines or nitrogen based compounds. All of this stuff must be in careful balance to provide the end user benefit. Throw in some dirt or moisture and the resultant stew can wreak havoc on any components that come in contact with fuel or fuel byproducts. Also, modern fuels are cutting compounds that add lubricity like metallic ashes for emissions reasons. The newer cleaners have either bio-based or non-metallic additives to give fuel some additional lubricity. This the the primary reason I use a "high maintenance" cleaner in every tank.