Oiled Vs. Non Oiled

I was asked by my dealer for he first time. Guess it's suppose to cut down on dust. I choose non-oiled for the simple fact it was cheaper.

The oiled stuff usually is best for stokers, keeps things flowing nicley. Water tends to rust things up in them.

What type of oil is used on coal? Petroleum based or vegetable? If vegetable based, can it turn rancid while in storage? I think I know where my used motor oil ends up.