I got a little side tracked from looking at how much oil I would need for my car and truck and trying to see if they would be the same grade, and I found this information about the biltema gearbox oils.
80W/90 mineral oil specs -
http://www.biltema.se/BiltemaDocumen.../36978_TDS.pdf
75W/90 synthetic oil specs -
http://www.biltema.se/BiltemaDocumen.../36997_TDS.pdf
The number look fairly close together with exception to the viscosity index