Is there an easy way to determine what type of oil has been used when I do my next change in my 03 496ho's? Bought the boat last summer and owner had already done oil change and not sure what he used.. I believe he told me straight 25/40 and that's what I've been topping it off with.. although I've read on here it prefers synthetic??

put in what you want and go boating. no damage will be done.

my $.02 if you want to use synthetic go with a 5w40, for conventional use a 15w40.

There are lots of good choices for that engine. The 25w40 is one of them. I think there is something about not mixing certain oils so just do an oil change and go! Don't use a cheap Fram filter.