gtx2 is 20w50 and mineral oil - i doubt it'd recommended. Usually use with old falcons and such, which were designed for thicker oils. Sometimes going from something to thick to a thinner and better oil makes cars run smoother. Improves fuel economy also.
I'm using fuchs 10w40 and happy with it. Might try something else next time. Was thinking shell helix ultra 5w40... price has shot up almost $10 to bout $57 now though!!!!!!!