Reply to neil:
Agreed with wopy and JM - you just need to clean the sump out first, and then after running it a bit, take a sample after it's sit overnight. Regular initial monitoring will enable you to catch any contamination. Did you check with John on what he ran in it and the status of the sump?
I just took my D2's right final apart. It looked pristine looking in the oil fill hole and even after pulling the final cover off to expose the big gear, but right there in the bottom was a pile of sludge. I don't even bother trying to second-guess anymore for stuff that's sit a while - I just disassemble and clean it out. Never found anything that was clean.
All the "old timers" I grew up with had the opinion to stay away from Detergent Oils in old machines. Several were still farming with Thirty's and a couple were still running Sixty's. Says a lot when they had such long life of the machines considering all other factors.
Detergent Oils have a very bad habit of breaking loose some "crud" that then gets to those places that hurt an engine quickly! ND is getting difficult to find here so Good Luck.
The Thirty I had ran 30W and often a gallon or so of Marvel Mystery Oil with it. It is still running!
I do not believe I would run 10W30 or other multi-viscosity Detergent Oils in the old ones - pre oil filter models especially, very poor at best air cleaner, etc. did not help keep anything clean.
Change the oil, best thing you can do and when it gets dirty, dump it. Cheaper than finding replacement parts or worse, not getting them!
Color of oil is not necessarily the indication of DIRTY oil, know your conditions of operation, load, etc.
That is my opinion, hope it helps -
cts