I have been using Amsoil SS 5-20 in my 2013 GT. This is my first report with 2,646 miles on this oil. I have taken the car out to the local road course (High Plains Raceway) this summer, and after reading this and other threads, looks like I need to bump up my oil to 10-30 or 10-40. The only issue is this car is driven in the winter when the roads are fine.