According to various parts catalogues, be they Vauxhall, Opel or Chevrolet, this "special" oil is nothing more than Texaco Multigear 75W90R GL4/5. I'm pretty sure you can find an alternative that not only equals this specification, but exceeds it.
Changing the oil every major service would be sensible IMO, what oil you use is up to you, but i'm not sold on the "special" oil argument. Go with whatever you're comfortable with, I think the problems with these transfer boxes runs deeper than just the lubricant.
Dulac said it perfectly regarding warranty, however, if you're out of warranty, who cares? Vauxhall won't!
Edited by: Dave501
2013 Antara 2.2CDTI 163PS SE Nav S/S Manual AWD in Smoke Grey