Hi can anyone tell me whats the best oil to use in my 2004 impreza wrx the oil cap dont say anything and ive been told and ive used it 5w30 but the letters at the end of it does that matter aswell like c3 or ag or af or a5/b5

Any good branded fully synthetic oil will be fine, and should exceed the manufactures specs,
As for brand all of us use Miller's oils but it personal preference really, has your car been modified in any way ?


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Slightly 

Just gotta decat or sports cat it then stage 1 remap it