Just sold my Civic. Purchased a 1999 Isuzu Rodeo with the 3.2 V6 Engine for a really good price. It has 76,000 miles and suffers from the famous Isuzu "Valvetrain Noise". I put lots of money and time in finding the correct oil to fix this problem. I also wanted to find the perfect oil for my new SUV. The rodeo was well taken care of by the previous owner. I started with 5W20 Castrol GTX flush driving approximately 500 miles. The oil was dumped and filled with Castrol GTX 10w30 with a Wal-Mart filter. The truck ran great. Valvetrain noise subsided a great deal and gas mileage was 20 mpg in the city. I ran this oil for 1,000 miles and it was dumped and filled with Havoline 10w30 with a wal-mart filter. This oil made the valvetrain sound even worse than ever. I was embarrassed to drive the truck. It was dumped after 500 miles and bought Mobil Delvac 1300 15W40 with a wal-mart filter. This oil also made the valvetrain extremely noisy and the truck very sluggish. My gas mileage went down to 15 mpg. I also only drove this oil for 500 miles and dumped it. I did a little more research and bought Castrol (since my Isuzu likes it) Syntec Blend 5W30 since the thicker oil was worse than thinner oil. I also fitted on a wal-mart filter. It has been 300 miles and the truck loves this oil. Gas Mileage is back up to 20 mpg and the valvetrain noise has subsided. Any reason brands of oil makes such a difference? And do you recommend me to stick with Syntec Blend 5w30 or try Syntec Blend 10w30 next? Or just go back to plain old Castrol GTX 10w30? Comments appreciated.