Since we are on this topic, I discovered a new use for penetrating oil. I bought a Stihl 390 P/C kit for an upgrade project. The company that packed the kit forced the piston inside the cylinder and wedged a chunk of the pin's plastic wrap between the piston and the cylinder. The piston would thus not fall out of the cylinder and was stuck.
I tried rapping it a few times and even used a hardwood dowel to press down on the piston through the spark plug opening, but that failed -- until I decided to try some WD-40. I sprayed that inside the cylinder from bottom up and top town, waited a few minutes, and then rapped it again on the bench top. Bingo! It fell out. Then I found that piece of plastic wrap, shook my head, mumbled a few words, and continued.
The upgrade then went smoothly and the saw ran fine with a lot more power.