Equally important is that if you are going to use a Full Synthetic oil, you must still change your oil every 3000 miles. Even full synthetic begins it's breakdown after the first 500 miles - and loses around 20% of it's viscosity after 2500 miles.
Yet, I still know a handful of jackasses that will use full synthetic, and then wait 5000-9000 miles to do an oil change. Their logic is that if you spend twice as much, it should last twice as long. I've tried showing these people charts and studies on webpages that show the breakdown of oil, but they refuse to get it.
On the other hand, these are the friends of mine who are leasing BMW's and LEXUS's and won't be putting more than 20,000 miles per year on their cars - in a 2 year lease. I just feel bad for the poor schlep who buys their cars after they are "certified" by the dealership.