Final Drive Gear Oil

[Joe]

so....as usual, got charged by Dealership $75 for a BMW branded 500ml (only ?!) bottle of their gear oil Hypoid G3 70W80

What are you guys using INSTEAD of this ripoff ??
For example- will a Motul or Mobil brand of Gear Oil 75W90 work ??

I can't seem to find another 70W80 weight....does that matter ??
What is "Hypoid" exactly ??

Thanks!!
Joe