Oxidation is the main cause of heater deposits, plugged lines and that thick black mess that used to be a liquid when you open the drain valve. There is no backtracking - once the Acid Number (the test used to measure oxidation) reaches 0.4 to 0.5, it is only a matter of time until the problems begin. And there are no cures - you have to start all over with new oil. Even changing it can become a problem because oxidation increases the viscosity, which, in turn, increases the amount of effort required to fully drain the system.
The best way to ensure a problem-free system is to make sure that the expansion tank/reservoir temperature does not rise above 140°F (60°C) during normal operation. Make sure that you have not left a warmup valve open. Startups will cause the expansion tank temperature to increase for a short time but these relatively short exposures generally will not hurt anything. If all else fails, put a nitrogen blanket on the tank.
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