Originally Posted by
PDR1
Why would the oil pressure drift down over time and need "readjusting"?
The oil pressure is set by the action of a spring pushing a piston into a seat. The only thing that would cause the pressure to decrease would be the springt relaxing. The spring (if correctly designed) shouldn't be loaded anywhere near its elastic limit and so shouldn't "relax" unless it actually fails (in which case it would need replacement rather than readjustment).
As the engine wears the clearences will increase, and that will result in an increase in oil FLOW, but as long as the pressure release valve is still being pushed above its seat there should be no change to oil PRESSURE. That's why we have an oiul pressure relief valve in the first place.
PDR
Since I've not flown that plane with high oil pressure, my information is not first hand and may be confused. My understanding is that pressure was not dropping through the single flight, but very gradually dropping over the hours, although I may get confuse that too. In any case, the owner is now convinced to ask maintenance shop to conduct oil system inspection and to get oil pressure back into green arc where it belongs to. Let's see how that goes. I am sure once it is back to normal everyone including me will keep a very close eye on oil pressure needle and not tolerating any instability.
And thanks again for lots of interesting and insightful info. It is true that many pilots including me do not shine on systems knowledge. For an average charterer like me it is not trivial to get insights, so knowledge remains at or below PPL/ATPL theory courses level as most of stuff gets forgotten over the years. Following this discussion I even wanted to buy IO-240 maintenance or troubleshooting manual for some extra education, but that seems not a trivial task to do. Without close contact with mechanics it's not straightforward to improve the knowledge either.