Nothing wrong with Castrol GTX. It is not synthetic, but it is a good oil. No engine has to run on full synthetic, its a matter of choice. True enough, "they say" the synthetics can be left in there for longer periods. But personally, i like to change my oil when it looks dirty, regardless on whether it is synthetic or not. Any good oil and filter will work. You cant hurt anything by using a conventional oil as opposed to a synthetic.
It all boils down to how often you change it. And how fat your wallet is.
2011 GT. Grabber Blue. Procharged. 3:73 manual.Billit LCA's. Corsa Extremes. GT 500 wheels. Racemaster DR's. T-56 Tremec. Dynotech DS . Centerforce DF. American Racing headers/high flow cats 560 rwhp.Donny Walsh tuned
2012 Pony Package. Race Red.3:31 automatic.BBK longtubes and catted X.FRPP Sport axlebacks. BBK throttle body. Dynotech DS. Brembo wheels and tires. Strut tower brace. Airraid CAI. Bama tuned.