Weight of the recoil spring depends on the ammunition you’re shooting. I use a 15# recoil spring in my P10C when shooting 9mm ammo that I’ve loaded to meet minor Power Factor (velocity x bullet weight / 1000). For stouter ammo like +P or NATO rounds I would use the factory spring.
One compromise is to use the 18 lbs spring.
Basic rule of thumb: if your empty cases are dribbling out of the gun and landing at your feet use to a light weight recoil spring. If your empty cases are being ejected 20+ feet away, you may want to use a heavier spring. I replace my recoil springs every 5K-ish rounds.
