It is my understanding(I am not a mechanic), that the differance between the wet/dry test, is that the wet test will tell you if your loss of compression is due to bad rings or bad valves. For example, if you do the dry test(should be done first) and your readings are 165, 145, 168, 163 (comp. numbers should all be w/in ten of each other)across four cylinders, you would add oil to the 145 cyl, and retest. If the compression stays the same it is likely a valve problem(burnt, tight, stuck, etc.). If the compression goes up, it is likely a compression ring seal problem.