Pretty much any high quality motor oil in 5W30 or lighter will suffice. The synthetics are better suited, though. I started using the Syntec as I used it in my car and some of the IPSC crowd swore by it. I no longer use Syntec in my cars as the "true" synthetics like Mobil 1 are pretty much the same price around here and offer MUCH better protection than the Hydro-cracked Castrol. Once my stash of Syntec runs out, I will use the Mobil 1 on my guns.
When you change the oil in your car (if you are a do it yourselfer) just save the drips in the oil bottles. It will not take long to accumulate enough oil to keep you going for a while.