Most motor oils will actually last much longer than the 3000 mile intervals that most folks use and like mentioned above the problem is usually with contaminents in the oils as opposed to viscisity breakdown. This being said, your oil filter is the weak link. oil filters, as most of you know, have a bypass valve built inside of them so that when they get plugged up the bypass opens up and what then? You guessed it, no filter what so ever. The bypass is actually there so you don't blow oil filters up, very messy and not good for the motor depending on where and when this happens. Most conventional oils will actually last around 7-10K depending on your driving habits, I don't trust filters though. I will change them every 3K just because. The things you have to remember are already mentioned here, contaminents in the oil, and viscosity breakdown. Conytaminents can come from all kinds of places too. Here in the desert, lots of dust, carbon build up from inside the motor, fuel bypassing the rings, this is more common in carburated cars, not to mention any additives you may use. I would consider most of them junk, Amsoil is a great product, so is Prolong and Marvel mystery oil, but I would not depend on any of them for a whole lot of anything. I know people personally who have tried to get warranty stuff from amsoil and prolong both and believe it or not, the prolong folks were much easier to work with. The amsoil people wanted an oil sample from before engine failure and then after engine failure, who keeps oils amples from all of there oils changes. I know I don't.

All this being said, there are some good additives out there. Like I said I like Amsoil and Prolong, both should work well in theory. Rislone also makes a good additive as does Morey's and Lucas. Most of the rest are junk and I would not waste time and money on. As far as changing your oil regularly, my advice would be to just do it. All the additives in the world won't be as helpful as keeping good oil and filters in your car, they just help, I would consider them preventative maintenance, helpers if you will, all helping the greater good of keeping your engine running longer, stronger and better. Even the additive folks will tell you to keep your oil changed at regular intervals if asked in a way that could come back to them.