I had this problem on a mates gc8 about a week ago. If you have an oiled air filter, save yourself some money, go to repco or autobahn whatever you feel like giving your money to and ask for MAF cleaner($10-20).
I would try that before you drop $100 on an 02 sensor. had this problem on my silvia as well whenever she ran rich whipped out my magic can and fixed the problem.

I don't think the filter being oiled or not affects whether you should clean it or not. They get dirty from time to time regardless.

__________________
A man in nature;

"All in relation to nothing. Nothing in relation to infinity. A central point to both, And infinitely far from understanding either."

Lol at Repco staff said .When they ask sir is your Wrx diesel or petrol when asking them for a part . Fuck listening to them for oil advice may as well get your oil at woollies if you listen to them . Oil quality is not going to matter so much if you keep driving it without fixing the problem it's only going to get worse then cost a shit load more it's like putting a bandaid on a giant gash .


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