I run 98 in my VX SS which is stock apart from a cat back exhaust, I mainly run it for the supposedly cleaner burning, the detergent package is supposedly meant to keep the injectors etc cleaner. My VX runs great on it, but I have been in the country where 98 has not been available and thrown in the odd take of 91 or 95 and had no dramas.
With the VX V8 the recommendation in the Holden handbook is 91 octane.