D2 dirty oil in main clutch compartment

I think the D4 is similar to the D2 in that there is not really a seal on the main but an oil slinger. See this thread about the same problem on a D4.
If you are not going to be in water, add a small hole with a cotter key in the drain plug to prevent filling the clutch compartment with oil

https://www.acmoc.org/bb/discussion-d72/24867-d4-7u-rear-main-seal#203031

It probably came from the engine, but to be sure, you can accurately check the levels in the engine and in the transmission, and you can also clean the area around the transmission input seal. Then run it again, and check. The blackness is probably from the crud in the clutch compartment, if your engine oil is not yet black

Unless you have cleaned this compartment in your pre use servicing , the slim may have been there. How much have you operated the dozer? And what was the temperature? Warmer temperatures just made it more fluid. I believe all the operators books tells how to wash the compartment. Get it clean and try it again. Park with it as close to level and check oil levels closely, after operating it several times. Also if you put the nose up hill it tends to weep a bit more around the oil slinger on the crankshaft.

Check to see if your oil has diesel fuel in it. The oil slinger can have trouble working properly if the oil is thinned by diesel and there is too much liquid to handle. I had this problem with my D4. There are numerous threads that discuss the problem of fuel getting into the oil. Just a possibility, something to check.

Bruce