Factory recommended that because they were sponsors at the time. Now i bet they recommend Shell for OBVIOUS reasons. In reality, Redline or Amsoil 70-90 is great. Personally have had great results with Amsoil, and NO i am not an Amsoil wannabee. Love Mobile 1 synth 0W-40 for summer oil... 20W-50 for the track... or 5W-30 for winter driving.