Question for engine gurus

Last week I noticed the oil pressure is around 10 psi lower than before and just in the green. The oil pressure was kind of low from the very beginning when I got the engine new and when I screwed the adjuster screw near the #2 cylinder all the way in.
Today I checked the sensor reading against an old fashioned pressure gauge in parallel and they both showed the same pressure. I also blowed compressed air through the oil pressure hose - into the engine. The adjuster screw is still responsive, oil pressure is lower when backing off the adjuster screw.
Some time ago I read there are different springs out there for the oil adjuster screw, can someone confirm this? Any more infos are very welcome, spring rate, part numbers and where to get them and how to install them. I?m also interested in the part number for the seal which goes under the fixture, it is about 1.5" diameter, looks like copper asbestos.
Any hint is welcome.

The engine is an IO-375 with CS-Prop, 450 hours since new. Just changed the oil and filter and filled up with Aeroshell 15W-50 as usual. Engine runs very well and burns almost no oil.

Sorry for the long post.

Numbers please

Hi, Can you post your numbers and what tap port you are getting your readings from please?
RHill

The oil pressure is just in the green, about 58-60 psi, cold and warm. The oil pressure is taken from the port right of the vacuum pump flange. There are two ports and I used the one which comes out straight at six o?clock.

Pressure

The first step would be to remove the oil pressure relief valve assembly and clean the ball and seat. Inspect the ball carefully to make sure there are no defects in the surface. The balls are relatively cheap, so if in doubt replace with new.
I'm not sure if the different springs are applicable to the adjustable valve.
Something is very wrong. With the adjuster screwed all the way in you should be around 90#, probably more.
The optional springs are somewhat of a mystery. You need to talk to one of the major engine builders for more info.
Dart aircraft in Waco TX area may be able to help with the springs. They are strictly a parts source but they have ECI, Superior and some Lycoming. I believe the optional springs are identified by different color paint markings.

The different springs are color coded and are used with the non adjustable style cap.

So I will remove the oil pressure relief valve assembly and check things out. I need to order spare crush washers and looking for the part number, I guess it is AN900-22 (inner dia 1 3/8") ??

Spring

I "think" you need the "white" LW11713 #11 Spring for an angle valve I/O 360, if this is what you have. And, clean the ball and seat. If just a small particle
of carbon is in the seat it will lower the pressure dramatically (had that happen). Time to cut open the oil filter too. Could be a problem lurking. Also the "big boulder" filter at the bottom of the case too.

Please, please, double check my work here.

Ok, I did remove the relief valve and found a small piece of metal on the ball, cleaned and reinstalled it. The oil pressure is up again and can even be adjusted somewhat higher (70+ psi). But no way to go up to 90 psi with the adjuster screw. Hope the small chip is a single piece, on the last oil and filter change recently the oil filter was very clean.
Have a look at the spring - does it look right?

Maybe not helpful but I had the opposite problem on my O-320 (Cherokee 140) recently. Oil pressure was at the yellow line anytime the throttle was over 2000 RPM. Confirmed the reading was accurate with a direct read pressure gauge as well.

Mine was not adjustable like yours. It had 3 washers inside the cap, which is the max. I removed 2 and my pressure at idle is not right at 60 with higher RPM right in the middle of the green.

I agree with others that you should look for other causes, which may require a full tear-down to look at the crank and associated parts.

Also, did you by chance switch from single weight to multi-weight oil (50 to 20-50)? That could explain the difference.

If your sump screen has been clean and the filter is still clean, I would install a NEW spring and fly. Next oil change sent the oil out for analysis.