The stock oil cooler system is a bypass system that takes a portion of the oil flow from the oil pump, runs it through the cooler and returns it to the pan. None of the oil that flows through the cooler goes on to lubricate the engine, it just returns to the pan. It only functions above 45-ish PSI oil pressure and the restrictor is in the system to provide resistance to flow through the cooler system. If there was no restrictor, you would loose a lot of your oil flow through the cooler instead of through the bearings and turbos. Its not a high-performance full flow system, but it works well enough for a stock-ish TT. The Nismo cooler unit is a waste of time. If you are serious about cooling the oil, put in a full flow system(not the one Z1 sells either)
