My O-300 engine has had an oil cooler installed sometime in the distant past. A very experienced IA today pointed out something which he thought was curious: an oil line comes from the top of the engine from the oil gallery and continues back to the oil cooler. The oil goes through the cooler, presumably under pressure, and exits back into the engine sump. His concern was that the oil being simply dumped back into the sump would reduce oil pressure overall in the system. Generally my oil pressure is very high at startup, almost redline, and within a few minutes settles down into the yellow arc. At full power it goes into the green. And then after a while at cruise, it settles back down at the high end of the yellow. This is a 1965 gauge, so I have no idea about its accuracy and I know it could be made to read anything according to how the relief valve is set. I'm not sure if this oil cooler arrangement is in any way unusual. There would have to be some pressure differential to force the oil through the cooler, no?