I have a diesel truck and the GB with twin Ford Lehman natural Aspiration engines. I've constantly viewed the truck as a test platform for the boat. As such I went through about ten years of testing additives and then I switched to oils.
The boat's hour meters had failed and been replaced a few times so your guess is as good as mine on engine hours. Keeping in mind my boat came from Boston to Los Angeles via the ditch it has hours. Lots of hours.
Everyone in 1994 told me "you can't put synthetic in an old engine"
DON'T PUT SYNTHETIC HIGH DETERGENT OIL IN AN OLD ENGINE!!!
I said, watch me.
I changed one engine to synthetic the other was Rotella fossil. Started them both. Major difference in smoke and sheen and thats immediately. after a trip to Catalina and back I changed the Rotella to Delvac1 and I haven't regretted the change ever since.
It's much more expensive and my next change might be a different brand of synthetic if I can't find Delvac1 reasonable.
One last note.
I replaced the engine in my Jimmy at 616,000 miles. I had a complete engine build, balanced, ceramic coated pistons, longer duration cam, etc plus a newer year block. I drove that on fossil for 16,000 miles and switched to Delvac1 synthetic. I was never real happy with it and I was burning oil more than I expected for a new engine.
Finally after many years and about 60,000 miles I changed to fossil oil. The first 3000 miles I went through three quarts. Changed it to fossil again. This is that change now and my engine has finally seated in. I'm not burning oil and I went up 1.2 MPG as well. I'm going to go one more fossil change then I'll switch back to Delvac 1.
BTW, I'm using Delvac 15W 40 I think. I have to check.