I use 5w30 Castrol Edge in my 15year old Legacy GT.

Over 5000km, my engine has never needed a top up of oil.

If you have a oil catch can, is it filling up quickly?

Also get a compression test for all the cylinders. Sometimes a single cylinder running a lower compression than the others due to what ever can feel and sound like a misfire.

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RS Liberty - Because WRX's are only good for parts...