Magnum 5.9L-- which motor oil/weight?

The PH8A is the comon regular size filter, and as far as I know the big one.
I know there is a shorter one people that run headers use.

Synthetics will find anyplace there could be an oil leak and take advantage of it, but obviously it's a good way to go for a new motor (More expensive though)

If you want a good oil at a decent price look into Rotella T 15-40
It has 1,200 ppm of zinc and is thin enough to flow well.
One gallon containers are about 14 bucks.

Castrol and Valvoline racing are really good oils also, but they don't have the zinc content that the older engines like.
Zinc was removed from oils for the most part in the 70's and was one of the reasons there was an upturn in cam and lifter failures (Especially solids)

There are a ton of oppinions on oil here, so get ready.