I'm using Motul engine oil and Millers gear box and diff oil.
To be Honest if it was an everyday STI I had, I would have no problem in running it on 10w40... I've ran loads of cars on Magnetec 10w40.. Without issue, and 10w40 from memory is what all Subaru dealers were using..,
I've come to the conclusion that once your putting something decent in that's all that matters, and someone's warm up or lack of warm up procedure is going to have a far more adverse effect on an engine than whether it's Castrol, Millers, Motul etc etc..
It's only when you have an oil temp gauge or safety feature built into Ecu that you realise just how long it takes to get oil up to working temperature..