Another alternative is to convert SAE 30 to the appropriate ISO grade and get an appropriate compressor oil in that grade. If I recall correctly, Shell and Mobil in North America do it that way, rather than having a simple ND 30 like Castrol. Castrol probably has oils rated by ISO grade too, but that's really not the issue.
Royal Purple, Shell, and Mobil all have synthetic, monograde compressor lubes, in addition to Amsoil, which was already mentioned. If it were me, and I were wanting synthetic, I'd be grabbing the Amsoil, since that would be most cost effective (for me at least) in reasonable quantities. Imperial Oil wanted me to buy 1200 litres or some other ungodly amount when I wanted one of their compressor lubes. I was willing to buy a fair amount, but not that much. For a couple quarts at a time, one would be hard pressed to beat the price and convenience of Amsoil, since the alternatives are harder to get in sensible quantities and at normal retail locations.
