The most accurate readings come from the testers that thread into the spark plug hole. Do everything everyone here has said, then thread the tester into #1 cylinder (snug, but not tight) and crank it like nine cycles until the reading has stopped rising. That's your dry number for #1. Repeat 5 more times in the other holes, write down your results then take an oil squirt can and squirt about 3 squirts into each cylinder through the plug holes. Let it settle for a bit, then turn the engine about 20 turns to spread the oil on the cylinder walls, and do a new course of compression tests as before. What the oil does is seal up the rings so you can read the valve seals. That is your wet test. It doesn't matter what the numbers are; If they are close and even, there's no problem. If both tests show a low reading on one cylinder, a valve adjustment is probably in order. Any anomalous readings need an explanation, but don't go manufacturing trouble. If it sounds and feels good, it is probably good.
