I was trying to pull a screw out of a part today and it just felt like I was going to strip the head. On the way to the hardware store to get some penetrating oil, I stopped at my car mechanics to return the loaner car and pick up my car. The only person still there was one of the mechanics working on his own car. I asked if he had any penetrating oil and any ideas about how to get the screw out without stripping the head. It is a Phillips head and recessed into a plastic part.
He said they use Aerokroil oil and "its the best stuff they've ever found".
After letting it sit for a few hours, I attempted to remove the screw. It came right out. Voila!
I purchased a can tonight. It's pricy at $20 on Amazon.
I'm sure there are a bunch of other good solutions but I was impressed with Aerokroil and the mechanics strong recommendation so thought I'd pass it on.
They have a bunch of other interesting products on kanolabs.com