Radar,
Man, you worry too much. My first 3 Bergs had that issue. I could run them around the block and the oil would look dark in the sight glass. When you actually drain it out and look at it, it doesnt really look that bad. My 06 has been the best so far as keeping the oil clean. I have no explanation for it. I have an 05 Husky that will go 8-9 months or about 5-600 miles of riding and the oil looks new coming out of it. Go figure.
As far as the diesel oil goes, some of the newer oils have friction modifiers in them that may not be that compatable with wet clutchs. Just something to think about. I have run Delo before and had good luck. Lately I just run motorcycle specifc synthetics.