Most would suggest to run the recommended oil for your engine as in the service book or oil filler cap. This is of course open to change depending on what sort of modifications and power you are running.
I think 5w40 is popular for most standard Impreza engines, that's what Scoobyworld would supply for example.
Mine runs 10w60 and is a standard engine, I run it purely to try and calm down the piston slap mine suffers with, although it doesn't make much difference...
The car runs fine although I wouldn't choose that grade of oil for it had I not got piston slap to live with.
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