I've used a product called Seafoam before, and it worked surprisingly well. It's an engine and fuel cleaner, put a 1/4 in your gas tank, and crankcase, and also throttle body if applicable. Give it about 15 mins to soak everything up and start your engine, and if your engine has alot of carbon build up, it'll be smoking like a chimney out the exhaust. Do this before an oil change, and my oils always been clean and stays clean for 3 to 5,000 kms. Ive only done it once, i don't think it's something you should do every month or even couple months, but it helped me with performance, gas mileage, and cleaner oil.. But if your engine Barely has any carbon build up, it probably won't do much.