what oil for compressor ?

Thanks for that, was just unsure as the sludge was really thick and have worked on some big compressors which ran really thick oil, serves me right for forgetting to service it, only 18 years !
Ttfn Glenn

Thanks, there's a toolstation near but I will stick with engine oil as I have a 20 litre drum and per litre its half the price
Ttfn Glenn

Finally got my Google head on, and did some research, now its clear as mud. Seems the usual is nd 30 meaning non detergent 30 grade but often Mobil 1 is listed as an alternative. It seems the reason for non detergent is to let the sludge settle and not be carried in the oil. Something to do with the sludge making the valves sticky, but presumably the Mobil 1 being synthetic dosent cause this problem.
Since my aching head cannot stand anymore confusion, I will stick with the 10/40 which is semi synth so should be semi ok, but change it a bit more often, like every year not 20 years. seems the correct oil is best but ordinary engine oil is only just second. Plus if the compressor does go phut there's no shame in it, and will give my an excuse to upgrade.
Ttfn Glenn

The reasons my head is spinning are, they say use non detergent 30 oil or Mobil 1, which is a multigrade detergent oil
Ttfn Glenn