Usually - it usually works just the opposite from what you stated. Overheated hydraulics was - and still is - a big problem with closed center systems. Not with open center.
Open center system constantly pushes oil at near zero pressure. The only exception is when a filter is partially plugged causing a flow-restriction. There is very little heat caused by flowing zero-pressure oil. If it's run through an open-air cooler - the flow helps to keep the oil cool, if anything.
A closed center system must work to maintain a very high line pressure - usually 2250 PSI. Even has to do it when cranking the engine to start it - thus the many starting problems and burnt starter motors on John Deere tractors. In theory - when everything is perfect in the tractor - the pump is supposed to reach the high pressure setting and shut itself off. In the real world, that rarely happens. Small internal leaks develop all over the tractor. They don't affect operation - except it can make a hot tractor very hard to start - and causes hydrualic system overheating after long use. In addition, since there is no real flow of oil when functions are not in use, oil coolers don't work very well. With Deere, two systems are used together. An open center gear pump feeds oil to the main closed center piston pump. Feed oil gets some cooling, but rarely enough.
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