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I used Castrol Syntrax - Its 75w/90 graded as per Subaru GL5 standards. Rear diff uses around 0.8 - 0.9 litres. Halfords sell this in 1 litre for £9.95
I tried Silkolene in the gearbox but it allowed the synchro to be beaten easily at higher revs (4th to 5th very noticable) Changed to Syntrax - cured!
JohnD