I'm gonna resurrect this old post from the dead, because I feel like it's one of the most debated concepts in engine maintenance, and some healthy discussion does us all some good. The primary difference between a 5W40 and a 15W40 oil is the cold temperature performance. At operating temperatures, theoretically BOTH oils perform the same. The advantage of the 5W is upon startup. Bottom line is 5W40 gets to the nooks and crannies faster than 15W40. Once you're steaming along at 200 degrees, both oils provide similar viscosity and lubricity.
I ran Shell Rotella T6 in my Honda CBR600RR sportbike since Day 1. We're talking high compression engine w/ gnat's-*** tight tolerances spinning at 15,000rpm at 220-degrees on the track. After a top end tear down at 30,000 miles, the cam lobes looked brand new, and after 3k miles of abuse each oil change still came out a translucent amber color... never burnt.
Besides all that, the greatest benefit I noticed from switching to 5W was how quickly the engine "quieted" down after startup. That 600RR is idling at 2000rpm on a cold start, with a hydraulic cam chain tensioner that relies on oil pressure to keep the cam chain happy. The slick stuff got in between the hard parts faster, and protected the same at operating temp as the 10W40 would have.
Read this article and decide what you feel most comfortable with:
https://www.bobistheoilguy.com/motor-oil-102/