Oil weight 5w40 or 10w50 ?

Not trying to start a debate about oil here but soon I'll do my first service myself on my 1290 SD. I see the book recommends 5w40 but my local shop carry's 10w50 so which one should I go with ? I'm located here in New Orleans which gets very hot a humid during the summer and winters are pretty mild here. Also would 4 liters be plenty and I'll take any other suggestions for my first oil change. I have always done my own first service on my Japanese sport bikes so what else should I be doing on my SD for the first time service, thanks.....