If you are driving a thirsty Porsche, you definitely do not want to route through Northern Portugal.
At present, petrol is up to €0,23 per litre cheaper in Spain and additionally, it is also cheaper to drive most of the way through Spain as, for the most part, the route avoid tolls.
Have a look at this article on my blog from February: http://bit.ly/a4HOL1
It will give you my full recommended routing. Just come off the A22 Via de Infante at Lagoa instead.
On final thought, remember to uplift fuel at Ayamonte just before you cross the Spanish/Portuguese frontier.
