high oil pressure

I'm flying a Pazmany PL-2 with 0-290D2B, 810 hrs SMOH. A year ago, due to low oil pressure, I added washers to the max number, and oil pressure rose to normal level. Was using PhillipsXC10-50 and oil pressure stayed in green without a problem. Recently changed oil and am now using AeroShell15-50. Oil pressure on start and climb out is in the high green. But after flying for 30-40 minutes at 2300rpm, oil pressure approaches the top of green. Oil temp remains in green. When power is lowered, oil pressure goes down to middle green. Question, is this difference caused by the change in types of oil? Might there be a problem with the gauge readings? Or any other ideas? Thanks.

I would remove the oil pressure relief assembly and carefully clean and inspect it. Looking for chunks of carbon or other foreign matter . Inspect the ball and seat for pitting . If all is good put it back together and test run. If all checks ok then fly it and watch it for a while. It may be that the oil was the reason for the change in oil pressure but I would feel better knowing that all was ok with the regulator. Have you talked to a mechanic about this ?..

It would be nice to know the actual pressure rather than "high green" or "middle green". Contrary to popular belief oil pumps don't provide pressure rather they provide flow and it takes a restriction of some type to provide the pressure (think putting your finger over the end of a garden hose). That should be the job of the oil pressure regulator. While not exactly familiar with the 0290 my previous Cherokee 140 with a 0320 ran with oil pressure of 85 psi (high green on the gauge markings) for 1500 hours until I sold it. I expect it's still running there. Having said that it's possible to put a temporary 'master' oil pressure gauge in the system to verify the actual pressure. As the poster above indicates I'd be looking at the pressure regulator.

Both of the previous posts make really good points. Put a decent gauge on the engine so you know what you have, even if it's only for testing purposes. High oil pressure is not any more desirable than low oil pressure. Too high oil pressure can float the back side of a bearing allowing it to spin. But more importantly, it takes horsepower to turn the oil pump. Excess oil pressure is wasted horsepower for no good reason. So find out what it actually has for pressure, pull the oil pressure relief assembly for cleaning and inspection and get it set up correctly.

-CubBuilder