Compression Test - A simple Guage costs from $30-60. Warm engine to operating temp. Remove coil wire. Pull one spark plug at a time, insert compression guage into spark plug hole, Hold throttle wide open and crank the engine over. Watch the guage and record the pressures at least three revolutions. The guage will go up each time. After recording each cylinder Pressures, compare the results from all the cylinders.
Leak down test puts compressed air in each cylinder and measures how much air leaks out through the valves and/or past the rings. Each cylinder needs to be on the compression stroke when testing. You will listen for air leaking to determine where it is leaking from.
I would suggest the compression test first. That will pinpoint if a problem is in the combustion chamber area.
You say it uses very little oil. How much oil does it go through in say, 500 miles? (1/4, 1/2, 3/4 quart)
