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Its called Castrol Syntrax Universal 75W90 mate,and you can get
4 litres of it fron this website delivered for 27 quid.
http://www.castrolclassicoils.co.uk/index.html
Paul
I assume you'll be doing a complete oil change as Syntrax is very effective in the Subaru gearbox (certainly from my experience after trying fully synthetic)
Syntrax is rated at 75w/90 which is exactly what is recommended.
JohnD