if i cast my mind back there was some uproar about one of the set of oils from vauxhall dealers being woefully inadequate. I think it was the accumulator/viscous coupling oil. or it might have been the transfer box.
iirc the GM oil did not like heat too much and seperated negating its lubrication properties, and i remember a mobil alternative or something along those lines.
any gen ? is the GM stuff now good ?