Oil pressure drop O-320

Okay,

I have a question for the engine experts: is the following normal or cause for concern? In the case that it's the later what should I be inspecting?

I have a rebuilt O-320 that just completed a 50 hour break-in. The oil presssure on the first flight was right around 80 psi at cruise. The oil (20W-50 Mineral oil) got changed at 10 hours and oil report was good. Twenty five hours later at 35 hours SMOH the oil got changed again and the oil report was again good. Right after the oil change I noticed that the oil pressure had dropped to 74 psi in cruise. This oil pressure reading has been stable for the last 20 hours or so, but at about 55 total hours on the engine, the oil pressure dropped to 69 psi. I had intended to change the oil at 60 hours total time (in another 3-5 hours).

The outside air temperature has changed in this time period but not the oil temperature in cruise, so that leads me to believe that the results are not temperature related.

The engine now uses very little oil, something like 1 quart in the last 10 hours.

All other performance of the engine appears to be excellent, good compressions, no oil leaks, runs good, etc.

I also checked that the oil pressure sender (van's) has a good ground, it does.

This oil pressure drop seems odd to me. The only things I can think to inspect are: 1. change the oil early and send it in for analysis to make sure nothing is destroying itself. 2. Check the calibration of the oil pressure sender by hooking it up to my air compressor.

Do any of the engine experts have any advice for me? What would this type of symptom normally indicate if anything? Would it be normal for the oil pressure to vary slightly 10 psi over the course of a break-in -or- what other items should I be inspecting? or checking?

I am reluctant to add a shim to the oil pressure relief valve to bring the pressure back up to 80psi as this seems be treating problem not the cause. If this is not normal I want to address any warning signs early on.

Sincerely,

RVPILOT2000

oil pressure

The first thing to check is the oil pressure relief valve ball and seat for contamination.

Check indication, if good, check relief valve for debris holding the ball off the seat. If that?s good, check filter for metal and suction screen for obstructions.
Good Luck,
Mahlon
"The opinions and information provided in this and all of my posts are hopefully helpful to you. Please use the information provided responsibly and at your own risk."

Something similar happend to me.

When I started flying my IO-360-A1A, after break-in, with phillips XC 20W-50, the oil pressure was about 65 psi at 180F.

20 hrs later, it was down to 58 psi. So at the next oil change, I cranked in the oil pressure relief valve 2 turns. pressure went up to 64. 10 hrs later, it was down to 58 psi again. So at the next oil change, I turned in the relief valve another two turns. Same story, 10 hrs later, it was back down in the 56-58 psi range.

What the heck?

So, again, I turned the relief valve adjuster in 3 turns, and this time, I safety-wired the nut through the roll-pin hole. Guess what...the pressure has been rock-steady at 70 psi now for 100+ hrs.

There is a lot of friction in the pressure relief adjuster, and I never saw one safetied before. But I think they should be safetied to keep them from backing out.

oil pressure

I had a similar thing with my 0-360, memory is a bit foggy on it but I'm quite sure I had to change the relief valve spring after just 10 or 15 hours of use, been fine ever since.
Jerry