Been using engine oil in my chain oilers for many years, and its miles better than the blue Scottoil which is really thin, and washes off in the rain.
I used to use recycled engine oil, but it REALLY mings your bike up, and I'm bone idle when it comes to cleaning!:
For best results in using engine oil in a scottoiler, you have to drill out the brass spigot slightly that attaches the feed pipe to the scottoiler body. It increases the flow slightly, as engine oil is slightly thicker, and you might have problems setting it up.
Whatever oil you use, as long as the chain stays lubed, who cares! You can rest assured your chain will last twice as long as the sceptics that won't fit one.