YOU see alot of people using synthetic blend oils, what they may not know is--the blends only have between 5%--10% synthetic in a quart of oil. That would be on an average of 1/3 quarts of synthetic to 5 quarts of regular. If you notice they never tell you on the bottle what the percentage is, but by using the Word -Blend -they make you think it is 50/50. It is a rip off. By adding two quarts of full synthetic to three regular quarts you get a good blend of synthetic at approx. the same cost as 5 quarts of the so call blended stuff. One other interesting thing----Castrol full synthetic is not a True Synthetic, It is a more highly refined oil than normal that the U.S. lets them market as a synthetic. In parts of Europe Castrol is not allowed to market it as synthetic. Mobile One and most others are full synthetic. This info comes from a person who worked for 16 years in a research dept. for a major oil company