OK, I'll add my own experience to this thread-that-will-not-die.
I just changed out some Rotella T6 5Wsomething - you know, the diesel stuff. I put in Royal Purple 10W40
Oh my flippin' gawd !!
It's beautiful.
OK, so I didn't listen clearly enough to the "5W is too thin" admonitions, but it was fine for a while, getting a bit difficult in changes eventually, and just a little roughness at higher speed/torque that just didn't seem right. So probably the step up to 10W (duh!) is probably a strong component, and an oil change to anything fresh is going to be good, but after putting in three bottles of purple in Minnesota, then driving the 360 miles to Rockford that stuff has got itself everywhere it needs to be. Damn, it's so good I'm thinking of trying it out on the wife next time ...
Changes are smoother, pulling away is smooth and spritely (I don't even want to know what I was doing to my gears with the other stuff - this is like an engine recon), and she is smoother than I ever remember (the Connie, that is, not the wife). I find myself at 75 before I even know it.
I'll try some Mobile one next time, but I'm going to be hard to convince that this Royal Purple simply isn't the best. It's the same price in my local store ($10.99 a quart).