You said there's no sludge that you can see. You might try pulling the valve cover, as you can get a much better view with the cover off. Typically those little BX engines are very easy to get at. If it's as clean as you indicate, there's no problems. My oil in my two Kubotas comes out as black as you state, yet under the valve covers they were as clean as a whistle. Don't let the color of the lue fool you; these old IDI diesel engines run fairly dirty and the oil is just doing it's job. And for the sake of adding to a knowledge base, for goodness sake get a UOA on that "old" fluid and post it up, please! I would love to see what a super-long OCI presents in terms of wear and soot loading.
I'd do a few very "short" O/FCIs with ST 15w-40 and inexpensive filters. Then a few "moderate" OCIs with the same. After that, all should be good. Try this as a suggestion:
1) 1 hour OCI
2) 10 hour OCI
3) 25 hour OCI
4) 50 hour OCI
5) "normal" OCIs thereafter
The hyd/trans fluid isn't as big a deal because there's no soot in the hydraulic fluid, so less particulate build up to worry about. I'd just to a full service (fluid and filters) on the trans fluid and it should be fine.