Hello- Now that I've decided on a viscocity for my 03 GT Mustang, I now have to decide on a synthetic oil. I do a lot of city stop-n-go driving and take many short trips. Which oil would provide me with the most protection Red Line or Mobil 1? I've heard that Red Line might not be API cert. That would be a major factor in my decision, for my warranty might not hold up if I use a non-API approved oil. Thanks for your opinions.