I looked up the Mobil-1 page and the link below says Mobil-1 "is compatible with other motor oils". I agree the motor should be flushed of the old oil, but in my boo, this involves adding a can of "engne flush" which is basically kerosene in a can, if it is approved to do this in a diesel. I did it to my 427s when I changed over a few years ago, just to get everything circulated and hot, so I could pump out as much old junk as possible. Once you remove all the oil, you could dump in a couple cans of cheap stuff into the sump just to help get the remaining fluid in the sump mixed into the cheap oil, and then remove that too.
http://www.mobil1.com/USA-English/MotorOil/Oils/Mobil_1_Extended_Performance_15W-50.aspx
I don't think the marina is suggesting a professional engine flush like the hot flush systems some people use. The marina may well have some experience that doesn't show up on the literature, so I would listen to them; Volvo may also have some requirements. In the end, you'll have to make up your mind what you want to do, but from where I sit, I would think you can add a can of "engine flush" and then change over to synthetic at an oil change with little to no prep.
I noted the Mobil-1 15W50 is rated for diesel service, and I'm curious what the marina would be using for your diesel. I know AMSOIL has a "diesel specific" formula they use for truck fleet use, and I think it's a better product than Mobil-1, but Mobil is a very very good oil. AMSOIL, by the way, would have to be drop shipped to your office or to the marina, because they don't have a sales network like EXXON MOBIL does. AMSOIL, however, produces a very high quality product and I think you should check em out for those diesels.
regards, hope this helps. (hey post a pic of that boat if you got one)
Regards, P
