Got given an old Craftman air compressor from maybe the late 80's? It's got what looks like a Campbell Hausfeld twin cylinder single stage pump. It has a serpentine belt instead of a v-belt if that gives an idea... My coworker said it wasn't pumping very well any more.
I ran it a few minutes and then dumped out some completely water saturated chocolate milkshake oil with extra clumps at the bottom. I don't think it's ever been changed. I refilled, ran it, and drained it a couple of times with some conventional 10w40 to flush it out. After confirming that it took an excessive amount time to pump up, I took it apart to find some crusty/rusty reed valves that obviously hadn't been sealing well. A little WD40 and some steel wool cleaned that right up and now I have another nice running air compressor. I'll drain the oil again and fill with compressor oil after I run it a bit to hopefully get the last of the gunk out of the crank case. A plaque on the pump says "ND 30W oil", but I'll see if I can scare up something better.