Originally Posted by Efthreeoh
Okay, take it easy, you haven't done any real damage here (even if you did start the car). From what you've written, it sounds like you have not started the engine and circulated the coolant; that is a good thing. The good news is that oil is lighter than water so it has probably just remained in the coolant tank and will not be pulled down into the system by gravity. So don't start the car.
DO NOT DRAIN THE SYSTEM FIRST! I suggest you get a shop vac (a vacuum that sucks up water) and suck the water/oil mix out of the tank. Then remove the tank. Then drain the coolant out of the rest of the system (i.e. radiator) using the DIY posted in the DIY section. Check the coolant and see if there is oil in it. Stir up the coolant in the catch pan you drained it into and see if it "milks up" (looks like white grease). If there is oil in the coolant that came from the radiator, then you'll need to remove the radiator and wash it out with dish detergent. There are instructions on how to remove the radiator in the N52 water pump replacement DIY. You need to use detergent to clean up the oil residue from the parts because regular soap will not work as well. You'd probaly want to wash the coolant hoses going to the radiator also just for good measure.
Clean the coolant tank out with brake clean or engine degreaser, then follow up with dish detergent (that will remove all the oil residue). Let it dry and make sure all oil residue is gone.
Reassemble everything and refill the system in accordance with the coolant DIY.
And finally don’t tell anyone else that you did this or you’ll lose all your “car-guy” creds. (Sorry, a little friendly flame here is appropriate...)
The guy put oil in his coolant tank, no offence OP, but do you really think it's a good idea he does all that?