Hi Martin
Personally, I'd change it, but I work for an oil company
. a 15w-50 oil is so thick when cold that it will take a long time to start flowing properly from cold and then too thick when hot, so won't flow quickly enough and won't protect as well as a 5w-40. Perhaps a 15w-50 would be okay in the southern US with that engine, but it all comes down to oil temps.
Cheers
Tim
Personally, I'd change it, but I work for an oil company
Cheers
Tim