Doug C:
But Amsoil thickens at 4000 miles.
Please define "thickens"
If Amsoil (any variety) thickens out of grade at 4000 miles in your car, there is something wrong with your car. That's not an excuse, that's a fact. In the same car, same conditions, I'll bet Mobil 1 thickens as much if not more, percentagewise. And yes the M1 may still be in grade, but thickened more.......somehow people forget where the viscosity begins.