Over the last 200 to 300 hours my oil "usage" has been gradually increasing. Previously it would run predictably at eight hours to a quart. Over the the last year or so it has dropped to two to three hours for a quart of oil, and in the last month to just below two hours for a quart.
When oil usage started increasing, we did all the expected things to try to track it down. There is nothing that goes out the breather, there is nothing on the cowl (or floor), and the exhaust was always clean. Cold compressions are all 75+, no drop on the wiggle. Borescope would find oil on all the intake valve stems, and there was occasionally oil in the cylinder on inspection. We did wobble tests on the valves, and there was some, but not more than you'd expect. Still, we are reasoning that there is growing and significant leak along the valve stems, and the oil is being cleanly burned in combustion, and disappears with little trace. We also considered that perhaps an oil scraper ring has failed, but no easy way to prove that out without pulling cylinders.
Lycoming specs indicate that this engine remains airworthy down to 1.4 hours/quart (really!). All things considered, my mechanic advised for the last year or so, that oil is cheap. So I have been flying along carefully monitoring the usage. As it is now heading down to close to paper airworthiness, I have decided to pull the trigger. Annoyingly, the engine is still running really well, producing great power.
Not knowing which, if only one or two cylinders are misbehaving, we are reluctantly going for a full top overhaul. (understand the risks, and mechanic is well versed in maintaining engine case pressure, and will likely go one cylinder at a time.) Going with factory Lycoming cylinders. Previous cylinders were ECI Titans, I have heard some disparaging comments on the ECIs (surprise that they lasted that long, etc.), but who knows.
