Deadman,
Is your tractor operating up to sufficent operating temperature, or a little on the low side?
Do you know the sulfur content in the fuel you are burning?
Both would make for dirtier oil than what is considered normal.
Diesels tend to get nastier looking oil than gas burners, anyway.
Many years ago, while taking an automotive class at a local college, we viewed an oil study from another college. It suggested oil actually gets better from use, not worse. The problem, and reason for us changing the oil, is the dirt, which gets into it.
During WWII, airplanes were put on an oil filter change/top it off and send it back out schedule, to conserve oil, and still maintain the airplane engines.
David from jax