Great thread. When trying to remove a stuck bolt,nut or screw, try tightening it a cinch first, then loosening it. This tightening breaks the surface contact and usually come apart easily, without stripping the head.

WD 40 is not for loosening stuck parts. It displaces water (water displacer 40). To free up stuck parts, PB is best as mentioned, but kerosene is the main ingredient you'll get from many of the commercial sellers to free up rust. Diesel fuel will work also.

When using heat to remove a stuck bolt or nut, you want to heat the item until cherry red and then let it cool by itself. What happens is the metal after cooling actually shrinks. Make sure you heat the right bit. you don't want to shrink a stuck nut onto a stud.
You are not expanding the metal to loosen it. A popular misconception.

Always wear eye protection, also ear protection and gloves when needed.

To wash down a greasy engine, I lay out a plastic drop cloth, and drive the car up onto it. Then I put lots of open newpaper down under the engine compartment. I use degreasers with a cold engine, and some stiff used paint brushes to help agitate the grease. I use brake clean to wash the gunk off, and it collects in the news paper on the drop cloth. When I"m done, I gather the plastic drop cloth with the newspaper inside and put the whole lot into a garbage bag and take it to the dump.
Usually I'll try to let the brake clean thinners evaporate a while before I pack it into the bag.