I have Castrol Syntrax in the Gearbox of both my V5 STi and my Girlfriend's MY98 Turbo but I would need to change the rear Diff oil in both. What is the best oil for the job??? Can I use Castrol Syntrax in the rear Diff as well as the Gearbox???

 

i am using MOTUL PA90 with LSD

 

Yes, you can use Syntrax. The lsd is a sealed viscous type which allows non lsd specific fluids to be used.
JohnD