I use Mobil 1 15w50 (usually pick it up at a yearly 2 for 1 sale, works out to be about $2.50/quart) and run it for between 10 and 15k miles. Every oil analysis I've done on this stuff over the years shows that at 10k miles the oil is at 80% or better (meaning it has a LOT of life left). I've only done a couple at 15k miles, but figgure the filter needs changing by then anyway (I always change the oil/filter once a year ~ if I make any mileage or not).

I've only got 16k miles on my T, with an oil change at 2k, then again at 7k (after moving, I figgured I taxed the engine pretty badly and it deserved fresh fluids).. I'll probably send the oil off to be checked at the 10k mile mark after the holidays. This thing is still at the full mark at 9k miles too... first 4-wheeled vehicle that's ever done that for me.

There is ZERO need to change synthetic oil any sooner than 10k miles... I've seen this time and time again. I'm 90% certain that 15k miles is safe and reasonable and willing to try going 20k miles sometime just to see what the oil analysis says.