Y'all have made some interesting and excellent points regarding oils and engine operation. There is another aspect - additive that you have not addressed, FAA approved Microlon. I think there are many people who believe that Microlon, when used properly, will also protect and prolong the life of an engine. I personally used Mobil AV-1 religiously in a NEW IO-550 in my Bonanza. I
followed Continental's recommendations to the letter. I changed oil every 100 hours and cleaned the filter/screen (Oberg) every 50 hours. However, unlike Dave's C90, my IO-550 only used a quart every 25 hours. At 541 hours TT since new, my IO-550 started making metal (12 hours after draining and replacing with Aeroshell 15-50 due to Mobil AV-1 being taken off the market). When the engine was opened up, there was 1/4 inch of sludge (gray lead deposits) everywhere. Metal was every where also, and I believe the Microlon kept the engine from self destructing until we got to an airport. My filter/screen has a bypass light, so with a new vibration and the light, we stopped and checked the filter. Our Coupe with a fresh O-200 only uses a quart about every 18 - 20 hours, so we may not add any oil between changes at 25 hours. We also have the El Reno filter and kept the screen. I believe the secret is frequent oil changes, and if you can, use unleaded fuel. OBTW Mobil paid for the IO-550 rebuild, and we are very thankful for that. Lynn Nelsen N99387 52FL
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