Apologies as I just saw your last reply. The airplane is n5507m, Walt Bowe’s old plane. The logs are away from me right now but I believe the last (only) overhaul was 1978. Weve sent one oil sample off to Blackstone and they gave us some things to watch and get back to them. After I got signed off in it, I flew it about 40 hours in 5 months or so. Prior to that it had been sitting for some time. It is a certainty that we will be changing the engine out in the next year or two, hence my curiosity in the 550.