AMS Oil is good stuff but a little hard to find (sometimes) and not the same as when it was first developed (I used to sell the stuff 20+ years ago btw). NEO is the only full ester oil stock unless things have changed recently (if you can afford it). Royal Purple is easier to find and has the market share etc. (I wish I owned some stock). As for "old" motors, I'd guess the percentage of untouched V6/3.8s is getting to be a rarity (my front cam bearing went out at 144K many years ago) and I was using 10/30 AMS OIL for the whole trip..
High mileage and going to a heavier weight base with higher mileage has been argued longer than "which came first, the chicken or the egg". My 02' F150 has 175k and runs better now than it did when new and I'm not about to argue with Ford's decision to designate 5/20 (also doing oil samples indicating no out of speck areas of concern) with Royal Purple which seems to be more than adequate and if it does give up the ghost tomorrow, I wouldn't have grounds to complain since Ford would most likely have a good laugh with this mileage, if I did >).
Use the oil you prefer and change the weight too, it doesn't concern me in the least.
I've been doing this since the 60's and will continue to be amazed at the advances being made with lubricants and doubt very seriously ANYBODY on this forum has more miles under their belt, having run more than 2.3 million miles with my last (Series 60 Detroit 400) Peterbuilt which was purring like a kitten when I sold it in 2001 and retiring from living in a truck (and I'm still in the business).