Quality synthetic oil can go up to 25K miles between changes. The filter will have to be changed more often. This also depends on how you use your vehicle, and what sort of load it is subjected to. Personally, I go from 5K to 7.5K miles between changes, depending on how I used my car since the last change.
If you change your oil every 2K miles, you are just throwing away money, unless you are really hard on your motor. If you are changing synthetic (quality synthetic) at anything less than 5K miles, you are throwing away your money. Quality conventional motor oil can go 4K to 5K between changes, but this depends on how you use your vehicle. Do you tow trailers, are you hard on your car, etc.
I only use conventional oil for a break in period, then switch to synthetic. Once I build a motor, I use the conventional for about 5K miles, but change after the first start-up/run-in, then at 200 miles, then at 2000 miles. A new car only needs to go about 3k to 5k miles before switching to synthetic. That allows the motor to break in properly first.
If you already have miles on the car, you should flush the motor when switching to synthetic. After that, it should be fine. On higher mileage motors, you need to check oil consumption after switching. This is due to piston ring wear, gasket seal issues, etc, associated with normal wear and tear with conventional oils. If you notice that you are using oil after the swap, switch to a synthetic blend, a heavier conventional oil, or a conventional high mileage formulation.
Additives are snake oil, and should be avoided at all costs.
If you want some further information, you can look here:
http://www.corral.net/forums/showthr...ght=oil+change http://www.corral.net/forums/showthr...ght=oil+change http://www.corral.net/forums/showthr...ght=oil+change http://www.corral.net/forums/showthr...ight=synthetic http://www.corral.net/forums/showthr...ight=synthetic http://www.scuderiaciriani.com/rx7/oil_filter_study/ http://www.amsoil.com/lit/lng_article/index.htm http://www.performanceoiltechnology....KeptSecret.htm