Altitude and pressure

I would ask if the Oil pressure goes back up as you descend?
I suspect that the Oil pressure is more dependent on the reduced Engine RPMs at cruise and the Oil thinning due to the increased temperature, which is probably at its peak after the 15,000 ft climb.
The thinner air at altitude, which offers less cooling, is probably balanced by the cooler air which offers more.

This all sounds mostly normal and expected.

Yes, in the decent the oil pressure is climbing as you go down. As for the RPM their is no loss of RPM up high, still much less than WOT. I’m using a constant speed prop and keep the RPM at 5200 until the decent,then I use 5000 RPM and start seeing normal oil pressure around 11,000’.

Hi Sam

Oil pressure in the dry sump affects your gauge pressure readings.  It takes surface pressure on the oil in the tank to help the oil back to the oil pump.  if you drop pressure, as in fly higher, on top of the oil then the reading will naturally go down.  As you note when you fly back down the pressure will increase.  This is also dependent on your oil system in your aircraft.  At about 10,000 ft you will lose about 5 psi pressure and this may be more if you have some restrictions in the system.  At 15,500 you would expect to see at least 7 to 8 psi drop from sea level readings.  

With extreme altitude in drones some are fitted with special systems to add extra pressure into the oil tank to compensate for this pressure change.  

Cheers