Good info here, answers all your questions.
http://wiki.seloc.org/a/Oil_Labelling_Explained
Castrol RS isn't a true full synthetic btw. Personally I use Motul 300v 5-40. You'll get lots of different opinions of what is best oil. Do some research, googling and make your own mind up. Worthwhile talking to Alyn @ AS Performance on 0191 410 3770 for good prices and info too.
Andy.