After following this thread, and pulling the left end casting off to check the vanes and oiling, I found that aiming a small fan at the left end and back side, where the oil filtering and muffler canisters are, that after running the 915 handpiece for 2 hours at various power levels and speeds, that the handpiece was slightly warm to the touch, but completely comfortable to use. Mine is located partially under a shelf and near the wall, so the air circulation is probably not optimal for for the convection cooling to work properly. Mine runs on Dexron ATF and the Hobbs meter I installed shows over 330 hours since I installed the meter. GOK how much time was on it before I bought it some 10 years ago.
After servicing the vanes mine has been running the 915 handpiece warm but not super hot as earlier. I am building a sound proof box with 2 muffin fans for intake/exhaust to see if it cuts down the sound and removes the nasty smell of dexron 3 ATF. I did notice little oil outside the left rotary vane housing on the bottom rim. Not sure why. Mine is on a wooden board on concrete floor with enough clearance around it for the air circulation. Will try the circulation fan tomorrow to see if it helps. Thanks