Since I got my new STI back in 2013, I've been doing oil change on my own, following the factory recommended intervals at 6000Kms or 6 months which ever comes first, using German Castrol 0w30 and factory blue filter, recently I changed over to Castrol 0w40(German). I put less than 2000Kms every 6 months but I still change the oil and filter once every 6 months, the car has 23000Kms on it after 4 and a half year ownership, which I started to feel it's such a waste of money, what are your thoughts on this?? should I continue doing the oil change once every 6 months, or should I extend the intervals??