Mobil synthetic oil

There is a ton of information and discussion (and BS) on https://bobistheoilguy.com/.

My preference is for Shell Rotella T6 Synthetic 5W-40. Mobil 1 might impress more people, if I were planning to sell the boat and display how well it was maintained. The Mobil1 is an excellent oil, one of the best.

That is the oil that you need to use for the common rail Man's and is recommended by Man. If you've been running normal oil in your 820s for 18 years, why change? Mobil makes the oil for CAT, MAN, MTU and many other manufacturers, it's pretty much what I use exclusively unless I use the manufacturers branded oil.

High-detergent synthetics sometimes clean out waxy deposits left over the years by non-synthetics, that has been blamed for oil leaks. I'm a fan of synthetics, but in your case I'd stick with what has been working. Multi-viscosity would maintain viscosity over different temperatures better, maybe worth considering.

I switched to Rotella synthetic from Mobil 1 because I'd heard many people claim Rotella stops engine oil leaks. My old trucks were leaking so bad, I epoxied my garage floor to make it easier to clean up after them. Leaks stopped with Rotella. No idea why. I've purchased other old trucks that leaked, leaks stopped with Rotella. Mysterious, but I'm sticking with it in my road vehicles. Rotella T6 synth is on par with the better synthetics.

Mobil Delvac 1 ESP 5W-40 Full Synthetic is the correct oil for new MAN engines. I'd continue to use what you've been using however unless you find info that it is the incorrect oil.

I agree with 30west. Going to a synthetic at this stage in the game could cause a lot of oil leaks. I would stick with the mobil dyno oil you've been using. But I believe you should be using 15w-40 in the Mans

I have a customer with a 2001 50' Post with 820 mans, he has 5800 hours running regular 15w-40 and has never had a rebuild and they still run very good.

That's all I have ever used in my CR900s. I have an oil sample run every oil change (annually about 150-200hrs) and they always come back and say you can run your engines longer before you change oil. Never do but that's what they tell me in their reports.
Since you have been using the same oil for 18 yrs I wouldn't change beside Mobil 1 Delvac 5-40 is about $32-38 gal depending on where you get it.