My bro has just done an oil change on his using Castrol RS which I beleive is a 0-40w oil. I told him this oil was too thin and he'd be far better off with Mobil1 Motorsport which is a 15-50w oil. He was adamant that this was the oil recomended to him for his car by the shop guy.

What's the crack? Who's right? What can be expected if I'm right and the oil is too thin?