I have run 10/40 oil in my nine for 13000 miles and it is just fine. I believe I would change that 15/50 out of something a little lighter, Unless of course you live in death valley or the eqivilant heat in your area.
A multigrade like that is not a problem. You have a 15 weight oil at cold temps that protects like a 50 weight oil when hot. It has additives in it to keep it from thinning out. It is not much thicker to begin with.
Your 15W-50 meets the viscosity specs . Just make sure the API Classification is SF/SG or SH/SJ or the JASO (Japanese Classification) MA. Also if the API Dougnut Label says "Energy Conserving" you need to change it out. The energy conserving products will affect clutchs and not in a good way. They were designed for automotive engine applications by having a different viscosity modifier in the additive package. Safe riding. 8)