My car is about to have it's 8th birthday. Seeing as it has been so good to me I've decided to treat it well as it reaches old age. I did some research on synthetic oil and people indicated that they liked it a lot better than normal oil. I had the guys put in 10W30 Castol Syntec into the car. I drove it for three months and changed the oil like normal. I head that your first run of synthetic should be a short interval because all the "crap" in the engine will get loosened up by the synthetic and drain out. I asked the guys in the pit to pay special attention to what color the oil was that came out. They said it was dark brown. It would appear that a lot of "crap" did come out of the engine. Now I've put in 5W30 Castrol Syntec and I'm wondering how often I should change it. With normal oil I'd change it every 3 months or 5000 km whichever came first. I've heard I can go 10,000 km on the synthetic. Is this true? The car is very happy now. Revs smoother, runs slightly cooler, and the idle is flat. At idle the tach used to bump up and down a bit. Now it just sits there dead steady. I just has my emission test done for my new sticker and the car passed with flying colors. In fact the test results were pretty amazing including 0 hydrocarbons at idle. So do you use synthetic? Plus how often do you change your oil?