Hi Jez,
I don't know what the PSD pump's like,I got a pretty cheap affair of e-bay called a Ucal,
suitable for petrol or diesel pumping at about 5psi but good flow rate. The thinnest oil I could find that it would pump easily was Redline 0-10,it's really purely for track cars and it's pretty costly. Lad at work had some lying unused and kindly donated it. I added 50ml of ZX1 an oil additive I already use for the G. You can get it in Halfrauds. There's probably nowt wrong with compressor oil though, I just felt it wasn't pumping very easily. You pays yer money, takes yer chance really.