It looks as if the tractor manufacturers are lengthening their oil change intervals. In the '70s-'80s a typical interval was 50 hours, which is what my diesel Mitsubishi calls for. I change mine more frequently, about every 25-30 hours, and even then the oil is pitch black--far darker than the oil would be in a gasoline engine at change time. For a diesel engine, 75 hours, let alone 100 hours, seems to be pushing it a lot, even with today's improved oils and engines. Diesels really contaminate oil; it's the nature of the beast.
That leads me to a concern. There is evidence that the car manufacturers are recommending longer oil change intervals not because engines and oils are supposedly so much better, but because they know people simply don't change their oil as often as they should. I know people who don't change their oil but every 10,000+ miles as they get around to it, and we're talking dino oil here. However, especially with low-viscosity oils such as 5W20 that many new cars call for, the cynic in me suspects that this is also a way for the car makers to make $ selling remanufactured engines when the original engines fail long before they should because of these longer change intervals.
Mechanics have talked in magazines and on computer forums about the differences they see inside engines between those that had 3,000-mile changes and those that had the manufacturer's recommended 5,000- to 6,000-mile changes. Usually the one that had the 6K changes is filthy inside with sludge, massive amounts of varnish, and other problems, and it failed long before the one that had 3K changes. Often these engines don't even make 100,000 miles, even though the owner did have the oil changed every 6K miles as called for.
And now the tractor makers are going down the same path. The cynic in me says that this is to encourage sales of engine rebuilds and replacement engines down the road, long before it should happen. It never hurts to change the oil a little more often.