5W40 is what you want for your S3, especially when remapped but there are different types of oils (thinner/thicker) within each viscosity so you want to make sure that it is the correct type you're using (needs to be 504.00/507.00 spec). Can you find out more specifics about the Nissan oil? Ester oils are the best out there of which a couple of examples are Millers Nanodrive CFS and Motul 300V.