The GTX that is GF-5 rated exceeds the performance requirements of GF-3 and ILSAC is backwards capable meaning if a oil is GF-5 certified it exceeds the requirements of previous certifications. So the GF-5 certified oil you use today exceeds GF-3 certification, you can use GF-3 oil if you can find it but good luck it is some old stuff but there is some around if you look hard enough.