Castrol non-synthetic is an excellent oil, but I don't recommend it for any import engines. Our research w/ racing shows that the base stock of the Castrol doesn't respond well to the oil wipers in our race engines, or the oil pickup in many offroad motors. This seems to be an import only phenomenon, as I know many 5.0 guys that LOVE Castrol. I'll agree about the Syntec,but I wouldn't but Amsoil in the same catagory as Mobil 1, and I wouldn't put Redline on that high of a pedastal. Mobil has some REALLY good synthetic, but I'm still not a proponet of the extended oil changes, even though I know that the synthetic have signinficantly better wear properties. I still think 6 quarts of oil and a filter are cheaper than a new motor. I've got a 1996 3.4 liter motor in my 4Runner that was broken in on mineral Mobil 1, then switched to Mobil Syn after break in, nothing but that since. That motor just passed the compression test 1 whopping point from factory. That's w/ over 170k on the clock. That folks, is essentially a new motor. How's that for durability?
That also said, I've seen various issues where higher mileage motors did NOT respond well to synthetic, due to sealing issues where the smaller molecules of the synthetic seeped past the older seals. Just some food for thought.