Hello all-- I'm reviewing an airplane for purchase that has a known long in the tooth engine - a Lycoming IO-540 with 1600hrs/18 years. I'm valuating it in a way that if I had to overhaul it the day after purchase I wouldn't be mad. That said, I'm really perplexed by what I see in the compression readings, comparing year over year: 2016 - 78,75,72,74,76,73 2017 - 78,78,79,76,76,80 2018 - 74,78,80,80,80,77 2019 - 74,66,64,68,74,75 If I look at 2019 alone, I'm thinking "she's going downhill fast" - but if I look at the trends they are all over the map. Conceptually I don't see any logic in how a cylinder (#3) could read 72 in '16, and then go UP in '17 and '18 to 80, just to then fall precipitously to 64 in '19. The owner provided an engine oil analysis that gave reason to believe there was a bit of life left in the engine, but ultimately, two questions: 1) Is there a good guide for interpreting compression checks over time? 2) If I want a best gauge of the engine condition is it compression, oil analysis, other? Much appreciated.