analysis reports. Also, got tired of paying the premium and dealing with having it shipped to the house. Running Mobil 1 15W-50 now. I can buy a 5 qt. jug at Walmart for pretty cheap. Rotella is also a good choice, cheap and available at Walmart. Maybe running at the track the RP 20W-50 would shine? For a street driven Z I personally don't think there's much benefit. FWIW, lifters seem quieter running the 15W-50 vs the 20W-50.