The diesels with which I have had oil change experience over the years always turned the oil black almost as soon as I restarted the engine, which I just thought was normal for all diesels. At least it made it easy to see the level on the stick. When we acquired our current boat with it's Perkins 6.354, I was amazed how the oil stays relatively clear throughout it's life of about 100 hours between changes. Granted, we don't run it hard and usually stay under a boosted rpm of 1650 & I suppose it's also due to the placement of the cooler and filter lines. Maybe everything drains into the pan at every shut down plus there's a plug at the back of the engine just over the oil pan that allows me to stick the vacuum tube all the way into the bottom lowest corner of the pan to get every ounce of oil out.
The only word of caution I can offer is my personal experience with synthetic oil. I read where some would say don't use it in older engines because it will weep past seals and gaskets where conventional oil wouldn't. When we got the boat, it had relatively low hours for a diesel- around 2500- and the bilge stayed as clean as a penny. Well guess what? A couple of seasons ago I was all set to do a fluid change and all the local store had was a synthetic blend. I gave it a go and all was normal until the next season when, after the seasons first longer cruise, oil appeared in the bilge. Lot's of oil! Thanks to Jay Leonard's post about the forward engine mount bolts being drilled into the oil passageway, I healed up most of the source. I still see oil weeping where we never had leaks in the past like a little at the head gasket, an access plate on the side of the block, and similar inconsequential but annoying places.
Coincidence? Perhaps. It is a 40+ YO engine. I can assure you that I'll drive to the next town to find conventional oil from now on, if I have to!