I fitted mine with an electric ball valve, so everytime the mains plug is pulles it releases the air from the tank, far faster than a normal drain one.
It is best to drain everday, I have never once oiled my air tools, though know I should.
I have never changed the oil in my compressors either as if doing it as often as they say, it is actually now more costy effective to just use it and abuse it then buy another, depending on size etc(50 litres in my case), they are so cheap now.
My sip is now 12 years old, the last 25 litre one was £49 at ALDI's.
tabetha