WD-40 is a good choice for this as well.
You can also try "tapping" it. Make sure you are using a good screwdriver (not plastic handle) and put it on the screw like you were trying to remove it, then give the handle end of the screwdriver a couple light but sharp taps with a hammer. sometimes the vibration will separate the bond that the internal rust has formed, if not enough to remove it perhaps enough to let the penetrating oil work.
I use Kroil now ,but used to use WD-40 and liquid wrench.