Here is some info on the best kinds of penetrating oils to use. Their purpose is to free fittings that are locked together and will not move. Often, this is due to rust but can also be from anything such as corrosion and gasket cement in the wrong place. The table below is reputedly from a machinist magazine of long ago where these penetrating oils were tried on some rusted parts. I am not sure how scientific the tests were. My own experience is with WD-40 and Liquid Wrench and I would give the latter higher marks. Kano Kroil has a very good reputation (I just ordered some to try it out).
No Penetrant 516 pounds
WD-40 238 pounds
PB Blaster 214 pounds
Liquid Wrench 127 pounds
Kano Kroil 106 pounds
ATF / Acetone 53 pounds (50-50 mix)
The ATF / Acetone mix is only good if the mix is shaken well just before use because the two compounds do not mix (like salad dressing). It is also very flammable, even explosive. For these reasons I would use the other commercially available compounds. Liquid Wrench is probably the best value for the money. For really tough jobs, use the Kano Kroil. WD-40 does more than just free seized parts, especially displacing moisture ("WD" = water displacing).