Less of the "you can", it's pretty certain "you will" unless it either has an automatic drain or you have incredibly dry air.You can get condensation lying in the bottom of compressed air receivers and they can be corroded internally if they've been neglected. I drained 30 litres out of one that had a capacity of about 50 litres.
I too have come to drain a compressor where the owner didn't know about this and he was rather surprised what came out !