What was found
20 MINUTES INTO A FLT, THE OIL TEMP WENT FULL HOT. A CROSS CHECK OF INSTRUMENTS REVEALED ALL CYL TEMPERATURES NORMAL. THERE WAS NO LOSS OF ENG POWER, HOWEVER, OIL PRESS WAS LOWER THAN NORMAL. THE ONLY WAY TO LOWER THE OIL TEMP WAS TO REDUCE POWER AND PUT THE AIRCRAFT INTO A GLIDE. RETURN TO THE AIRPORT WAS MADE. THE VERNATHERM VLV WAS REMOVED AND UPON CHECKING IT IN HOT WATER, FOUND TO BE DEFECTIVE.