Most stand along liquid coolers are closed loop systems - messing with that is a bad idea.
In addition, mineral oil is not nearly as useful as you might thing. It has thermal quality closer to insulation than to a coolant, and keep this in mind. It's a LOT more viscous than water, which means a pump rated to move water at a given rate will move mineral oil a lot slower... this means that you're going to have hot mineral oil sitting on top of your CPU.
TLDR: Do not do this. Very very bad idea unless you want to fry your processor.