Boat is a 2003. The fill cap says 15w40 (b/c they had a deal with Valvoline marine oil). PO ran M1 15w50 for many years. It had a fresh fill of M1 15w50 when i bought the boat in spring of 2017 and I ran it that year.
I was going to stick with M1 15w50 until the manufacturer emailed me put 5w30...