Best high mileage oil? Well there is only one synthetic (perhaps Group III type synthetic) high mileage oil that I know of and I would rate it the best: Valvoline Maxlife Synthetic. Good zddp levels; a boatload of calcium for detergent, anti-acid, and anti-wear; and a nice slug of moly (around 300 ppm). O'Reileys (if you have them) is running a special on Maxlife Synthetic for well under $4 a quart.
Now if you want to go with conventional oil in high mileage, I really like regular Maxlife, which has about 12-22 percent PAO in it, but if you want really high zddp get Citgo Ultralife.