there are only 3 things that determine if an oil needs to be changed. Contamination. Additive Depletion, or Breakdown of the base oil. Contamination is controlled by the oil filter, the better the filter the longer it will last. Some over the road trucks using bypass filter systems run hundreds of thousands of miles before changing. The bypass filter systems arent cheap, but they do filter out just about ALL contaminants. Additive Depletion is basically the detergents added to the oil, they get used up after a period of time and can no longer keep the engine clean on the inside. If both of those are controlled, then all you have to worry about is the base oil breaking down, and losing its lubricating ability. Conventional oil will last about 3000-5000 miles, synthetics like Mobil 1 and others you can buy at the parts store will last about 10,000 (now mobil 1 claims 15,000) and high end synthetics such as Amsoil, Royal Purple, Redline, etc, will easily last 25k miles. I dont know anything about RP or Redline, but i do know that amsoil has enough of an additive package to last as long as they claim, otherwise they WOULDNT CLAIM IT!! they have a warranty to cover things, and if they didnt know the oil would last that long they wouldnt have the warranty. the biggest issue with extended drain is the FILTER, the filter must be changed at its corresponding intervals, so like an STP or Fram filter woulds till have to be changed every 3-5 k miles, even with amsoil oil, a Mobil 1, Hastings, or K&N filter would last 7500-10000, amsoil has a filter that claims 12,500 but that may be a simple convenience factor that it goes twice into their 25000 mile program. and for those with a bypass filter, the oil could last up to 50k miles or more. truckers that use the bypass filters just send a sample of the oil in every 50k miles to get it tested to see if they need to change it. i have documented proof of a truck that went 409,000 miles on the same amsoil oil, and when broken down has LESS wear and tear than if it had used convetional oil and changed every 5k miles
I am not pushing for anyone to do an extended drain of 25k miles, i use amsoil myself and still change both the oil and filter at 7500 miles, but i dont use Amsoil for the extended drain, i use it for the superior qualities of the oil itself.