I use 5w-30 in my '99. Thats what Ford recommends, so thats what I use.
As for longevity, like you said its a crapshoot. Personally, Ive got 156,000 miles on mine.
Ive seen a few E-Series vans with Triton engines that had well over 300,000 miles (I recall one that had over 500,000 on it), so Id say that durability is not something you should be concerned with.
Change the oil every 5,000 miles, change the air filter every 12,000 miles and your engine should just about last forever.