I did a quick search using separator and this thread indicates that the M20 is used on Skymasters
http://www.337skymaster.com/messages...ight=seperator
STC list includes 337's
http://m20turbos.hypermart.net/pdf/M...ligibility.pdf
The easiest way to calibrate your dipstick is at oil change. When you add new oil you know how much you have added so do the calibration at that point. Why mess with the old mess. I have found that my dipsticks are different. Front engine reads about right but the rear reads low so I was always adding when I didn't need to. If you have your level at 6 or below and you are still getting oil on the belly something else could be wrong.