I have been using a mixture of charcoal lighter fluid and automatic transmission fluid, about a 50/50 mix. Seems to be far superior to any other commercial product and much cheaper. I use a spray bottle with an adjustable nozzle so it will really "reach" out there. Obviously you want to keep the fluid away from flame or sparks, try to clean any visible threads and use high pressure air to clear away any debris or rust residue, spray the stuck part, wait, spray, wait and then gently try to move in both directions, a lot of the time a sharp rap or tap on top of a bolt will aid in removal. Good Luck.