P-Dude,
Good info in that link.
I use AMSOIL in my newer, tighter engines, because it is far superior and the extended intervals cuts down on one more maint. chore that I don't need. I'll go 15K miles before changing oil, but at 7.5K, I pull off the old filter and put a new AMSOIL filter on (loses about 1 qt. in the process). AMSOIL (and synthetics, generally) tend to "creep" thru seals and gaskets much more so than conventional oils. Something like what penetrating oil does. So on many engines, if you make a change to synthetics, you'll begin to see weeps and leaks here, there, and everywhere. Also, AMSOIL has a program where you run an external, auxiliary-type filter (one of theirs, naturally) and just routinely send oil samples in for analysis, and as long as the analysis checks out, their oil is good to go indefinitely. Many over-the-road trucking firms do this with excellent results over millions of road miles.
For me, at 25K miles/yr., that protection and extended life makes up for the 4x price differential. Not counting the 3 mpg increase I get with it.
BTW, I've read some other studies that put Mobil 1 at worse than most 79 cents/quart conventionals as far as protection/life goes. I keep as far away from M-1 as possible.
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