well, my personal opinion says stay away from pennzoil products for a couple reasons:
1. tho i have no personal experience with this, i know that those who have rebuilt engines run w/ pennzoil often tell horror stories of how nasty internal engine components look w/ some kind of nasty build up.
2. they own jiffylube. the way i see it, if your company is willing to employ unknowledgeable people to promote your product in a way such that your company developes a bad name, then you don't care enough about the quality your product either. i had the misfortune of working for jiffy lube and i can say first hand, they don't give a sh*t about the employees - only the bottom line.
don't know where you've been getting that consensus about synthetic oil and mileage, but it's wrong. if you read the bottle it will tell you that you still need to change it every 3000 miles. you can run dino oil for 6000 miles too, and you won't feel much of a difference... for a while. the additives in either fluid are designed for a certain mileage, it's just simply the base that's different. synthetic oil has a nice quality of sticking to engine parts better than dino oil, so everything stays lubed nicer.
highmileage oil is a joke. they'll try and sell you on it over and over again. high mileage has additives in it which are designed to expand seals such that it will supposedly stop leaks. rarely have i heard success from this, and there's no reason to keep running it after a while - the additives which expand the seals will have done all they can at some point, as there comes a time when seals won't expand any more.
if you really care about your engine, you'll run either mobil 1 synthetic, or amsoil.