you want the thinnest oil you can get, I used to use compressor oil in mine. If you are using a fuel pump then it's not designed to move anything thicker than petrol.
As for pressure all I can say is "some". I had a mechanical gauge connected to mine so I could check the pressure didn't drop to zero. I recall it only ran at a couple of PSI most of the time, you want the gauge connected to the oil in line, the out line will be little to no pressure.