[papaduck]

Penetrating oils, some are better that others and a home brew is best.

They are below, as forwarded by an ex-student and professional machinist.
They arranged a subjective test of all the popular penetrates with the control being the torque required to remove the nut from a "scientifically rusted" environment.

*Penetrating oils ........... Average torque load to loosen*
No Oil used .................. 516 pounds
WD-40 ..................... ....238 pounds
PB Blaster ................... 214 pounds
Liquid Wrench ...............127 pounds
Kano Kroil .................... 106 pounds
ATF* - Acetone mix..........53 pounds

The ATF-Acetone mix is a "mix-your-own-brew" -- 50 - 50 automatic transmission fluid and acetone. Laquer thinner can be used in place of the acetone.

Note this "home brew" released bolts better than any commercial product in this one particular test.

Note also that "Liquid Wrench" is almost as good as "Kroil" for about 20% of the price.

 

[HOTLAP]

Seafoam Deep Creep is an exceptional penetrating fluid also, the best I have found plus you can use a torch without worring about it catching on fire.
I have never tried the ATF/acetone mix but I'll have to give it a try.

 

[maxwellxx]

the the ATF/acetone mix is good if you can get the part in the mixture. Say soak the part in a gal container.

down side is acetone is very Flammable, so be careful if you use a torch on part later.

acetone will flash off fast so it does not hang around long.

1st. if it just has to come apart i use the torch if possible
2nd. i use ATF/acetone mix
3rd. I use Kroil

if one of these will not get it apart, then most times you better be looking for another part.