In over 40 years of experience twisting wrenches, I've found that PB Blaster is the best penetrating oil out there. You can find it at better auto parts stores, Amazon, Walmart, etc. Just be careful not to get it on any painted finishes you care about, or in your eyes.
If the screw is cross threaded (or the wrong size or thread), penetrating oil isn't going to help much at all. If that's the case and you don't want to replace the saddle, you could flip it over and carefully file the screw down until it's the desired height.
__________________While the firelight's aglow, strange shadows in the flames will grow, 'til things we've never seen, will seem familiar.