Philip,
I picked up a set of OEM 15" x 6.5" alloy wheels from an '85 300ZX Turbo a couple of weeks ago and they actually have a 30mm. offset (not 20mm. as in your list). The offset number is actually cast right into the outer side of the wheel. Kinda screwed me up since I will need some pretty thick wheel spacers to fit them over the Z32 calipers.
Bob