I'm running my Castrol Edge for 15,000 miles and every 3,000 miles my car burns about 1 quart. I have 3,000 miles on the oil already and refilled with 1 quart of Castrol Edge. However, it's getting expensive because one quart is about $8.25 out the door. There's a local Amsoil dealer will sell me Amsoil Synthetic 7,500 mile formula for $6.50. What kinds of oils is the Amsoil comparable with? $6.50 is an awesome price for a Synthetic.