lower oil pressure in mid teens

[toomany]

i've been noticing a drop on oil pressure in the bravo when i climb into the mid to upper teens.  for instance today i flew from roa to lns at fl170 and pil pressure was on the low green limit which i believe is 60psi.  on the ground it's consistently 80psi and drops to about 70psi around 10-12k.  i remember seeing a similar post on here but can't find it.  any ideas/input are appreciated. 

we did install another gauge as a monitoring gauge due to previous erroneous readings by the factory oil pressure transducer - we still get the intermittent needle shake on the factory gauge but monitoring gauge remains solid.  both gauges show the same drop in oil pressure at altitude.  oil temp is consistently 180°F and engine settings are always around 28.5-29 inches at 2350 rpm's, leaned to 1625-1635 on the TIT, fuel flow 17.3 - 17.9 gph.  

feedback is appreciated.