I agree with Fred, In our case, a CS prop converted to fixed, the
acessory cover on one of our Pawnees has leaked, and it behaved
exactly as he described..

As far as exceeding the oil pressure redline, we have this problem as
T described. Oil temp needs to be over 140 to allow full power. I
warm up 6-10 min, then partial power takoff ~2300 as part of the first
flight of the day check, by the time I land, the oil temp is up and
full power is possible without exceeding the pressure redline.

aerodyne