Question

What will cool iron faster, water or motor oil?

Answer 1

Water, especially if the iron temperature starts out above the water boiling point. Motor oil will boil at a higher temperature. The higher specific heat and lower viscosity of water should aid convection below the boiling point. These are my opinions; I have not looked up any numbers nor plugged into heat transfer equations. I suggest you do that. You will need to compute Nusselt, Grashof and Reynolds numbers.