Oil is your car's lifeblood.
Dirty oil, less protection.
I change mine as soon as it loses that golden-brown look and starts heading to dark brown/black.
That's about every 4500 km, with regular oil.
BTW, with synthetic, if you don't change it out yourself, are you suuuure you're really getting synthetic??
Yes, I'm a cynic. I even mark my oil filter somehow to make sure I get a new one every time I take the car to the dealership. (caught a Toyota dealer trying to pull that scam once)
