Spray some oil in the cylinder and see if it goes up that will tell you if the rings are going bad some what. Also, check the rocker lash. Furthermore, take the compression hose off and valve core out and hook it up to the air compressor and blow air into the cylinders and see if any is coming out of the intake or exhaust with the rockers off or at zero lift on the cam. 5-10% difference is ok 15 is kinda getting high.