Sorry about the short message, but I'm on a bit of a tight schedule today.
Castrol Magnatec Professional is the one they buy in Bulk and is actually just cheap oil in disguise, don't go for it.
Due to regional differences, your manual would probably be the best bet. 5W30 is a suitable oil for those temperatures, I've personally been using it for a while! Castrol Magnatec 5W30 is awesome stuff.
But I see your car is modified, that may also be a factor to consider when choosing your oil.