The “Fleet” oils are generally rated for SJ/SL and CH-4/CI-4 (S=Spark/C=Compression). The additive package in these oils are generally more robust than their SJ/SL only counterparts. They are generally more shear stable, have a higher capacity to buffer acids (higher and more stable TBN), and have a stronger detergent package. In a sense, they have everything a marine oil needs. High and stable TBN to deal with rich running marine engines (gas that is) and a corrosion conducive environment , shear stability for those often and extended high rpm WOT runs, and great deposit control.
The Merc oil you mentioned in your other post on the Additive forum is mix of two oils, a 25 weight oil and a 40 weight oil. Very weird. Volvo Penta, who uses the EXACT same marinized GM gas motors as as Merc recommends a straight 30 weigh synthetic oil…go figure. I’ve talked to the Volvo Penta tech people about oil in my 5.0 motor. They thought that 15W-40 or 30 weight “Fleet Oil” was a good fit. George Van Pares (master marine mechanic) from Boatered.com uses Shell Rotella in his shop.