A couple of things here to let you know about. First thing is that the oil may have leaked out of the front pump seal, behind the pulley, since you've stated that the oil came out with the entire unit resting on the pulley overnight. Possibility there to consider at least.
On a cast iron Tecumseh compressor, there's a bolt right on top, maybe 3/8-1/2 inch size, within a recessed hole (making the top of the bolt basically level with the top of the cast iron housing) that is the fill hole for oil. They also will put dye in there manually too, if they are about to hook it up to a machine for charging, testing for leaks. The dye will show up with a black light, if I remember correctly. The oil is a special type/viscosity specific to air compressors and I found it at the local auto parts store (only sold it at the time & location in quarts), so I donated the rest of it to the mechanic, once he had enough for our repairs and recharge. This was about 6 years ago.
At any rate, I would not bother to put any more oil in there if it's leaking. The oil mixes with the refrigerant at a specific mix rate, (R12 from my experience and background - R134 may be different) but they will add that in when they add refrigerant during a recharge procedure, once your system is checked for leaks and ready to charge back up to operation again. When ours was down for the up-teenth time years back, I took the belt off and ran it without, just to be safe. Our car has the factory smog and power steering and I believe I had to find a different size belt temporarily to pull this off, until I was ready to fix the compressor leaks we had and charge it back up. Hope this helps some.
