The pictures below are of the sump plug on a VY 3.8L V6 Commodore. As you can see, there is a "lip" on the top edge of the drain hole. This lip sits within the diameter of the valve, so the valve hits this lip before the valve can seat properly.
We currently do not offer an extension/adaptor for this valve, so the only possible solution is to use a thick aluminium or copper washer to space out the valve to clear the lip.
The valve to use with a suitable washer is the EZ-8.
Thanks to Aaron for the pictures.
VZ 3.6L seems to be very similar. A thick washer would be necessary to clear the slight recess. Again, EZ-8 (24mm x 1.5) would be the valve to use.
Thanks to Matt for the pic.