As 2O2F said, theres alot of diff. opinions here. A good quality oil is hard to beat, Rotella T is a good one, has high film strength, zddp to boot. Shell Rotella, Mystic, in the 15-40 range is good. If you loose any between oil changes, just depends on how much you drive your car, add Lucas Oil Stabilizer, and your motor will purr for another 50 years. Since your motor I-6 and has a longer stroke....I'd stay with what you got. Motors are funny, they get used to certain things, and oil is one of them. But definitely add the Lucas, it'll help.
Alot of the different weights depends on the usage, the type of motor, and the heat involved. In Miami, you'd want a 15-40 or a 20w50. In Canada, you could get away with a 10w30. Arizona or New Mexico, in a desert climate, you could use a straight weight.
It all depends. I use the racing oils almost exclusively, they have the stuff the old oils used to have. Everybody was so dam worried about the zddp messing up smog equip. and causing birth defects....the world is a whole lot better off with it, than without it. You don't see them doing away with teflon, its even been found in our DNA. Nice....at least I won't stick to the inside of my coffin....