An old lead mechanic showed me I was reading oil analysis reports wrong

I've been doing oil analysis on my own GA plane for about 4 years now, just watching the wear metals. Last month an old mechanic at Oshkosh looked at my reports and said I should be tracking the viscosity and oxidation numbers far more carefully, because those tell you about oil breakdown before metal shows up. Does this match your experience, or do you think it depends on the engine type?

3 comments

3 Comments

the_rowan2mo ago

@elizabethg18 has me feeling dumb for missing the obvious stuff, guess my "precision flying" doesn't extend to reading reports.

elizabethg182mo ago

That old mechanic was absolutely right in my case. I spent two years just watching metal levels on my Lycoming until I started tracking viscosity and TAN numbers, and that's where I caught real oil breakdown happening way before any metal showed up. Once I started seeing the viscosity drop off around 35 hours, I adjusted my oil change intervals and it made a big difference in how clean the engine stayed. It really depends on how you fly too, since short hops cook the oil faster than long cross countries, but the numbers don't lie.

elizabethn561mo agoMost Upvoted

Don't get me wrong @elizabethg18, I respect that you put in the work to track those numbers, but aren't you just chasing the same problem from a different angle? Viscosity and TAN changes are still telling you the oil is breaking down, not why it's breaking down in the first place. I've seen engines that kept their numbers great right up to 50 hours but still had sludge buildup because the real issue was moisture from those short hops you mentioned. Seems like tracking one set of symptoms over another doesn't get you any closer to fixing the root cause, which is usually how the engine is run and what the oil is actually dealing with. I'd rather spend that time on more frequent visual inspections and compression checks than trying to game the oil change interval by a few hours.