I would just take spark plug out and pour in a ounce of the fuel mix you are going to run the engine with. Crank it over several times to get mix all through engine parts. Put compression gage in place....crank engine like you would to start it. That should give you pretty accurate reading. Put plug in and check compression by hand propping engine. Remember oil /fuel mix you just used is the same mix you are going to use to start engine. The use of any oil with high viscosity will give false compression readings. Good luck Capt,n