Do NOT rent from SIXT in Portugal. The SIXT cars in Porto and Lisbon are old, high mileage, need service and dirty. Despite their advertising they have no new BMW or Mercedes rentals with SIXT in Portugal, Lisbon or Porto. Their remaining (3) Alfa Romeos are being returned this month!
From local discussions, the problem seems to be because the SIXT franchisee in Portugal cannot get financing for new cars. SIXT HQ should help solve the problem - but in the meantime we really cannot recommend SIXT in Portugal. Other travellers we met in Portugal June 2014 rented from GOLDCAR RENTAL enjoyed fresh new cars. (One we saw had 18kms on pickup.) Note that GOLDCAR have no branch in Porto. The best advice is to rent and return in LISBON. Happy to provide more details of our experience on request.