Some interesting background to the SB classification here **LINK** and here **LINK** .
Essentially its an older relatively low additive oil formulated for engines that spend significant periods stopped and have widely varying load / speed ranges. So it has a decent amount of anti-corrosion and anti-condensation properties. For all practical purposes anything up to, but not including, SM works just fine on older automotive engines made when earlier designations were current.
420 is the viscosity. presumably in Saybolt SUS rating at 100°F as nothing else I've found makes anything approaching sense. Straight SAE 30 covers about 380 to 600 SUS at 100°F so presumably would be a modern equivalent. Basic hydraulic oils are popular for this sort of relatively undemanding application but do make sure you have decent amounts of anti-corrosion and anti-condensation in the specification. If going the hydraulic oil route note that ISO grades are specified at 40 °C so are closely comparable to SUS at 100°F although the numbering is quite different.
One thing to be careful about compressor oils is "off-duty" cycle. Many compressor oils are formulated for industrial devices that run most of the time. Some oils can be unhappy if left to stand for long periods. The special oils for Hydrovane compressors are a case in point where its important that the beast be got up to temperature when run so as to drive out any condensation.
Personally I just shoved a decent brand SAE 30 automotive oil in my Atlas Copco KE 2 with an oil change when I thought about it, probably every 3 or 4 years. The Atlas wasn't new when I got it 40 odd years ago but its been going fine on that sort of service regime ever since. I've sold it 4 times and taken it back 3 times so far when the current owner decided to copy me and go Hydrovane for peace and quiet in the shop. Perhaps I'll look at the valves next time it comes back. Or perhaps not!
Clive.