I had this same issue a couple weeks ago at the Lake of the Ozarks. It was the first time I needed to add oil and I figured I'd just beebop on over to the marina and pick up some Penzoil 15w40. No dice. No one carries it. So then I went to just any 15w40 but back pedaled when I realized most 15w40's are diesel oil. I run Mobil Delvac in my Duramax. I started reading as much as I could find before I dumped it in the LSV. There are equal parts information on NOT running diesel oil as there are FOR running it. What I found was for that much disinformation, I wasn't going to chance screwing up my motor. I called 4 separate marinas and spoke with their techs. All of them, told me not to run any type of diesel oil as it had detergents that could harm my motor (5.7 GM block/Indmar). They all independently told me to switch to any marine or racing oil with a 25w40 rating, stating that all Merc stuff had switched to it.
That's my .02¢ but there are a ton of forum members here, and on other sites that have been running Rotella for a long time with no issues.