Gear Oil: Questions & Answers

Hi.
In VW vehicles with heavily stressed transmissions, a special oil called G51 is used.
The standard oil is called G50.
The viscosity is the same for both oils: 75W-90.
But what's different about the G51, and where else can you buy it?
except for VW, where half a liter is supposed to cost a whopping 15 euros.
Is there perhaps a comparison table available?
Here are the part numbers again.
for G50 G005 000 05
and G51 G005 100 A1

Hi.
I saw that Volkswagen has published the transmission oil specifications under the "Tips and Tricks" section.
Does anyone know if the G51 oil is available from sources other than Volkswagen?
or what makes it special.