The SAE 15W50 (or 10W40) refers to the viscosity of the oil at the upper and lower temperatures. 15W50 will be effective at slightly higher temperature than the 10W40 but the 10W40 will be more effective at slightly lower temperature than the 15W50. If you live in the more temperate climates of the UK, to put it technically, it’ll make cock all difference. Andy
Either way, it ain't going to blow up on you if you used it for a year. I prefer to run my Dukes on 15w50 fully synthetic whenever I can. However, when times have been tough and in bikes I don't ride that often, I'm happy to put in 15w50 semi. In the early days of my Ducati ownership, I ran my Monster on 10w40 fully synthetic for a few years. I still have it and it's still fine! There are genuine advantages to the thicker oil in certain circumstances and I personally prefer to use it. However it's far more important to change both the oil and filter regularly than to lose sleep over viscosity and brand IMHO.
