Firstly, congratulations on your purchase and welcome to the forum.
Have a read here, these threads will give more info:
http://www.350z-uk.com/topic/71469-the-advantages-of-synthetic-oils-over-mineral-oils/
http://www.350z-uk.com/topic/10747-16-important-oil-questions-and-answers/
http://www.350z-uk.com/topic/7192-buying-oil-some-basic-advice-from-opie-oils/
http://www.350z-uk.com/topic/77-faq-what-engine-oil-should-i-use/
http://www.350z-uk.com/topic/94871-the-big-fluids-list-add-your-suggestions/
OpieOils is one of the trader here in this forum. Worth PM him and see if he has any forum discount or anything on offer if you wanted to buy from him.
As for Castrol, i'll leave it to other forum members who have good experience on them to comment I personally use Motul or Fuchs and have been changing oil every 5,000miles or every 1 year which ever comes first.