I work at Jiffy lube, here we recommend oil to be changed every 3000 miles or 3 months. Although synthetics do say they last longer lets say to 5000 miles, we still recommend every 3000 or 3 months. 3 months being because even if you dont drive within the mileage other factors do break down these oils, such as weather. Here in florida, its very humid and hot.. and these factors do tend to break down the additives in the oil faster than being in a cool state.
But I change my oil every 4000 miles with pennzoil full synthetic. I wouldnt go any further than that, because after then you wouldnt really know if your oil is in good shape, and could keeping fresh oil in you engine harm it?

 

My MS6 gets fresh Penzoil Platinum every 3000 miles, because the car sees a LOT of 5 mile trips to work every day, and only accumulates about 600 miles per month. That's much harder on any oil than just getting on the highway and driving all day.

The other reason is because of the thermal protection it offers for a turbo, and the lack of deposits left behind by a quality synthetic. I can justify the cost becuase I do my own oil changes, so it still costs me less to change full syn every 3000 than it does to have my oil changed by someone who doesn't even care about my car, using conventional oil. I take full advantage of Pep Boys rebates on this great oil. $1.99 per quart, and I have two cases (so far) stashed at home. Will probably go pick up some more this weekend.

 

I use only Amsoil synthetics (by far the very very best synthetic ever made - they cost more, they are so worth it). I use Series 2000 racing oil in my 2006 MazdaSpeed6. I change the oil filter every 5000 miles and drain a sample to send to the lab for comprehensive analysis and add a quart (approx). I'm on track to replace the oil at 25,000 miles which is well within the oil's life cycle. I'm at 15,000 miles now and just did an analysis at 12,000. The oil had not begun to break down and was cleared to continue being used.

And I live in Phoenix Arizona... where I drive my car like a banshee from you-know-where at all times and we experienced 38 days this year in excess of 115 degrees. So, that's demanding conditions if there ever were any and it does just fine. (Had a battery crack in the heat and corrode my wiring harness... but no oil issues, lol) Find an Amsoil dealer in your area if you're doubtful and they can provide you with all the testing comparisons done by 3rd parties to demonstrate the 'break down' cycles of Amsoil verses all the leading brands.

NOTE: You cannot do this with cheap synthetics like Penzoil and you can't push it this far with even the higher quality synthentics like the nicer Mobils, etc. So please don't try it thinking all synthetics are the same.