Since last year I have been using Mobil-1 Extended Performance 15K miles, 5W30 oil in my van. The bottle says it is guaranteed that the oil will not break down within 15K or 1 year which ever come first.
Like to hear some opinions about after how many miles the oil should be changed. I have been doing mine between 8K and 9K. As the oil come out, I feel it in my fingers and it seems quite slippery still.

I just started using Mobil EP 5w-30 @60k and plan on changing it at 70k. This is the first oil change I've done using this oil since the auto store was out of Mobil 1 5w-30 and only had this one available, I was a little skeptical about the 15k so I planned on changing it at 10k mi. I too would like to hear others opinion on this, thanks for bringing it up.

Check out the www.machinerylubrication.com (http://www.machinerylubrication.com) web site. This site has some very good information pertaining to synthetics and so on. I would at least change the filter during this extended oil change. I would also send oil samples to a lab during and after the long duration.