Croc

how often should i change it if i do quite a lot mountain and atocross driving?

Reckon

^At a minimum, you should go with the severe driving interval recommended in your owners manual. A full synthetic being used for mountain and autocross driving will probably not last 8-10k miles (depending of course on the harshness of the environment and driving). You could always do a UOA (used oil analysis), but to be safe, probably around 4k miles. A good synthetic will go at least 8-10k miles when driven 'normally.'

I'm using a synthetic blend and plan on changing every 7k miles with normal driving.

I'm sure some more knowledgeable oil folks will post up some other answers as well.