Fuel loses it's octane over time. But, too much fuel definately shouldn't cause pre-ignition/pinging. Has the car battery been flat? The ECU may not be running right now too (?)
Don't mean to whore up ur thread Pete, but I hear Barron is in the process of fitting the 4.375 GTS4 diffs to his 32 GT-R. Will be interesting to see the results...