Oil pressure drops with increasing RPM, but still stays well into the green

On startup many oil pressure readings start higher than normal for your plane, but usually come down as the engine warms. If your pressure at idle is okay, but drops significantly as you add power and then comes back up as you back back towards idle then you have a poor ground. Either check your grounds and tighten them or replace the one for the oil sender.

Could you define "significantly"? Is that anything more than a needle twitch, or a drop of several bar?

What are your thoughts on the core question? Is it safe to say that oil pressure should never go down with increased RPM?

On start up some see 60-72 psi on the oil pressure, but starts coming down fairly soon as it warms up. Many normal oil pressures seem to be 45-58 psi. Many times the oil pressure sender has a part in this and the reading may not always be 100% accurate. Having a good oil flow volume is better than pressure alone. Some engines oil pressure may be displayed on the panel at 45 psi, but if it was really tested may be 50-55 psi. This isn't a big deal and has been going on for decades. Some of today's pressure senders are definitely better than they were 30 years ago.

Back to your question. Many years ago I had this idle rpm displaying a normal pressure (50 psi) in a Flight Design CTSW plane (all carbon fiber). The more throttle you gave it the oil pressure would go down to 22 psi. When you went back to idle the pressure came back up. The pressure followed the throttle. I did a mechanical oil pressure test alongside the gauge on the instrument panel and found the pressure was actually fairly steady at higher rpms and didn't really go down to 22 psi. So I did a few test. So I took an Ohm meter and checked the wires. All seemed to be okay, but it wasn't. The meter said the ground was good, but I took a separate wire and bypassed the regular ground to the gauge. All problems went away so I replaced the ground wire.

So now when people come to me and say their oil pressure drops low as throttle increases and goes back up at idle I tell them try either try tightening the grounds or bypassing the original ground wire with a new one. Seems to always work. Some oil pressures senders like VDO are resistance type senders so a good ground is important.

I"m stilly murky on whether it should never happen. It's helpful to know a scenario where the gauge is wrong, but should oil pressure never decrease with increased RPM?

Here's video I took of momentarily increasing rpm while the engine is cold. The pressure drops from 4bar to 3.5bar, and then when decreasing speed back to idle it climbs back up to 4bar: https://imgur.com/a/CM5F4vR

58 psi (4 bar) to 50 psi (3.5 bar) When it's cold can be normal. Does it do this when all warmed up at operating temp? Does it stay fairly steady in cruise operations? This may or may not be a real pressure change. You'd need to put a mechanical gauge in line (so you see both gauges) and try that same test.

You'd need to rule this item in or out to make sure. Depending on the type of oil pressure sender you have it could be a resistance issue in the wiring and or grounding.