Even with engine motor oil, the motor still seemed to struggle some, so I took it apart and cleaned and relubed the bearings with Nye oil, which is a good lightweight oil. FWIW, here's the motor:
Based on this motor, does anyone care to guess what year it was made? After disassembly:
Note the metal removed from the rotor in two spots. Howard Industries took the expense of balancing their rotors. The brass/bronze bearings have felt oil-pads around them, making me suspect these bearings are not sintered. But I have too little experience with these to know for sure. I saturated both pads with Nye oil.
To my disappointment, the stator coil-assembly has no key to position it correctly, and the holes in it are not for the long screws you see in the photo. I aligned the stator by carefully matching tiny marks left over from manufacturing. Thinking about how these things work, I believe accurate alignment is not critical, but I still think an alignment key/pin/tab would be helpful.
Important:
After reassembly, the motor will be stiff. Here's why, and how to fix it. Each bearing is a sphere with a hole in the middle, so they rotate for alignment, in the same axis as the ball in a ball-valve. As a result, after reassembly, the bearings have been rotated into alignment, but that alignment-rotation has high friction, so there will be residual friction exerting force on the shaft, making the shaft stiff. You can remove this friction by forcing complete and accurate alignment of the bearings by placing a flat-blade screwdriver on the shaft, by the bearing, and tapping it with a hammer to rotate the bearing into perfect alignment. Do this again at 90 degrees to rotate the bearing in the other axis. Do this at both ends of the shaft. Now the motor will spin with lower friction. I hope this description made sense; it's much easier to show with pictures, but I didn't think to photograph that step.
The motor now raises and lowers the enlarger head without struggling.
Mark