If it's locked, you're probably not going to have much luck on the same day. You may need to soak it and come back in a few days after it's had time to soak in... or even repeated applications over the course of a couple weeks depending how bad it is.
There's lots of "favorite" brews for soaking the cylinders- Kroil/PB Blaster, Marvel Mystery oil, kerosene/ATF mix, etc. The common denominator is that they need time. And when you try, work it back and forth with a breaker bar, gradually increasing the distance you wiggle it. And don't force it too hard CCW, you'll just loosen the crank bolt.
