I had a look last night, to see the condition of the plug - it looks good, so I'm guessing a ring spanner will work no sweat (provided it's not done up too tightly).
I managed to find another problem while looking underneah -
http://forums.justcommodores.com.au/showthread.php?t=53987 - I'll have to get that sorted later.
So.... the parts needed are:
1 x 19mm ring spanner
1 x 5L bottle of your chosen oil
1 x Funnel for pouring it in
1 x Oil catch thing for collecting the old oil
I can reach under the car to the plug, so provided the spanner isn't too long, it should be possible to do this without raising the car.
The next question is the oil filter - how do you get it off? From above the car, or below? It looks like you have to work the oil filter around some of the hoses to get it in & out?