Oil is the life blood of the MINI Cooper engine, and it performs two main functions; to lubricate the moving parts of the MINI Cooper engine and to remove heat from the critical parts.
Oil itself is not stable and changes with temperature, the hotter it gets the less viscous (it gets thinner) it becomes and the opposite (get thicker) when colder. By design the MINI Cooper engine caters for the these changes in temperature; when the engine oil is cold it uses a small heat exchanger that warms up the oil using the water system which is quicker to heat up than the oil system. Once warmed up the oil is cooled by the engine sump, the heat being transferred to the sump casing and then cooled by the airflow under the MINI engine.
Why fit an oil cooler?
Changing the specification or use of the MINI Cooper can cause overheating of the oil. For example, as the engine RPM increases every 1000 rpm it require an extra 3 times the cooling! Thus a MINI Cooper that is used for track days or high speed road use can exceed the oil design temperatures. It is for this reason that the German specified MINI Coopers have oil coolers because of the possibility of long distance high speed autobahn driving.
Fitting of sump guards (while great for protection) or front spoilers and body kits (great for looks) can restrict the sump cooling, but again not good for your oil temperatures. Lastly increasing the power output of your MINI Cooper will increase the combustion temperature and as the MINI Cooper S uses a system that sprays oil underneath the piston in an attempt to have the oil absorb some of the heat, this results in even more heat into the oil which in turn must be controlled.
So to run a high power Mini Cooper on a track day with sump guards fitted is not the smartest of ideas–unless you have an oil cooler fitted
What happens if the oil gets too hot?
As said hot oil becomes thinner, losing its film strength. This film strength and oil pressure is what stops bearing surfaces from touching. If they do then metal is removed, clearances widen and pressure drops leading to increased wear and ultimately engine failure. Also the piston temperature increases and detonation can occur especially in tuned engines running altered timing and boost pressures.
Modern synthetic oils are very good with high temperature but can leave varnish deposits and block oil-ways within the engine so it is always best to keep your oil cool within its design limits and typically below 110 degrees C. On a fast run the MINI Cooper can get to over 120degrees C on a standard car.
For a long lasting performance MINI Cooper it makes sense to fit an oil cooler and regularly change both oil and filter Mini Mania highly recommends using Mobil 1 fully synthetic oil.