Senecio:When I was younger growing up in Australia the standard only went as far 15+. Back then it was noticeable that some 15+ brands were more effective than others but you would only find out through trial and error as the labelling only allowed 15+ to be claimed. As above it was later raised to 30+ and then again to 50+.
I was doing a photoshoot in Arizona some years ago and the guy I was with gave me a tube of SPF15 to put on. I asked him whether this was an undercoat or something...