Originally Posted by
JohnD
From experience it could be an expensive mistake to use full-synthetic fluid in the Impreza box. It works for some but there's also quite a few who have had issues with the synchro performance. It's wiser to stay with a good semi-syn. fluid such as Castrol Driveline (Syntrax)
By all means use a full-syn. in the rear diff.
JohnD
Castrol driveline/syntrax definately, ive tried a few fully synths in my classic and the syncros are affected by using them after trying a few different ones including millers/mobil/silkolene syn 5 ended up using castrol and love it.Initially with the syn 5 for a while were ok then noticed the change gradually getting a bit more notchy if ya like (bin here before had usual prob with 5th gear syncro)changed to syntrax-never been any trouble since an that were ages ago.