A new wrinkle to this age-old problem of accurate compression test readings--

When in doubt---- the automotive style/ spin compression tester will give a reading that is useful to compare and determine if a marginal cylinder/s are ready for replacement.

All A/P's and I/A's know that many factors (rocking prop/temperatures/adding oil) will influence differential compression tests.

These factors are less when spin testing.

P.S. I have been using MMO for several years, and it seems to be making a big difference. I operate LOP and the dryness of my exhaust impressed me that I needed to try an additive.  When my cylinder's reach 1600 hrs (and I think they will) I will be willing to make a firm statement.

Charlie K.