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Here's how the stock VW oil system works. Back when the engine was designed, the only oil available was single grade oil. The engineers made a simple assumption, low oil pressure means the oil is hot. This was reasonable because oil pressure drops as it gets warmer. Maximum pressure occurs with cold oil because it is stiff and resistant to flow. Hot oil flows easily and drops pressure. So they assume that low oil pressure = hot oil, and high oil pressure equals cold oil.
There are 2 separate oil control mechanisms, the first is pressure control, which is done by the flywheel piston. When pressure is "excessive", the piston is forced down and oil pressure is relieved via a bypass port back to the sump. If you have a HUGE pump (30mm or larger), even when the piston is down it still won't bypass enough oil because the relief port is too small. So if you are careful you can increase the size if this relief port, but this is not necessary if you only use 26mm pumps like we do. Putting in a stiffer spring will increase the point at which the pressure will bypass. This usually happens with a cold engine, you can see it on the gauge when you hit the bypass point, all of a sudden the pressure drops from ~50psi to 30 for example. It will recover when the piston closes again. The cooler relief piston operates in a similar manner, but it senses the pressure drop across the cooler. If the oil is cold, there is a lot of pressure drop across the cooler (cold oil is thick and tough to move). If the pressure drop is small, "the oil must be hot", and the piston remains closed and routes as much oil as possible to the oil cooler. Oil pressure is NOT affected by the cooler relief system, it's sole responsibility is for sending the appropriate amount of oil to the stock oil cooler. If the cooler relief spring is replaced with a stiff one, the oil will be routed to the cooler before it gets warmed up, and the engine oil will have a tough time reaching operating temperature. We feel that oil is too cold until it goes >180F, 200F is better. Anything from 200-230F is fine and nothing to worry about. There are a lot more VWs running around "too cold" than properly warmed up, which is also why you should ALWAYS use your flaps and thermostat!
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