What will cool iron faster, water or motor oil?
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.
Water will cool iron faster than motor oil.
To determine what will cool iron faster, water or motor oil, we need to consider the specific heat capacity of each substance. Specific heat capacity is the amount of heat energy required to raise the temperature of a substance by a certain amount.
Water has a higher specific heat capacity compared to motor oil. This means that water can absorb and retain more heat energy before its temperature starts to rise. On the other hand, motor oil has a lower specific heat capacity, so it requires less heat energy to increase its temperature.
Given this information, we can conclude that water will cool iron faster than motor oil. When placed on a hot iron, water will absorb heat from the iron more efficiently, causing the iron to cool down at a faster rate compared to motor oil.