Well guys.... about a month ago, I went in with 20w50 Valvoline synthetic in my 5.0 Mustang.... Problem is here in Tallahassee, temperature does get down in the high 20's..... The 20w50 made the Mustang real hard to start, and the oil pressure would take several minutes to reach its maximum!! I switched to GC 0w30 and oil pressure hits its max within 5 seconds.... Fully warmed up, the GC 0w30 produced 22psi of oil pressure... Valvoline syn 20w50 produced 30psi Amsoil 10w30 produced 20 psi Valvoline 10w30 produced 18psi I really like this German Castrol 0w30 I might even keep it around for the summer! or should I add a quart of 20w50 to thicken it up a tad for summer?