I've been using Redline since 1998 in my vehicles.
I've only run one oil change of full up 15w-40 in my truck, and my completely unscientific opinion is that it was awesome. I gained 2psi of boost just by changing oil
In the end, I just can't justify $30/gal in the crankcase when the Cummins will last forever with Crisco in the pan. (pardon the pun).
I also have had very good experience with Royal Purple and Amsoil as well.
In a racing application where oil can be the difference between winning and not finishing, I'd put Redline at the top of the list (along with Torco).
But in a street vehicle, it's probably overkill in the engine.
In any case, Redline in the axles is a good idea. Have you ever used ShockProof heavy?? Dang, that stuff is SUPER thick! It's like pink gooey honey! It's a NIGHTMARE to clean up too-- it doesn't dissolve easily, it sticks to everything, etc.
But Shockproof Heavy is probaby the "toughest" gear oil money can buy-- perfect for the pullers.
I've actually considered trying some ShockProof ultralight in my NV5600.
Be advised that the Shockproof line of gear oils contains less friction modifier: 3% vs the regular 5% in their other gear oils. So you may get some diff chatter on the SP oil.
If I were made of money, I'd have Redline in every possible crack and crevice. But I'm not, so I only have it where it seems to work the best for affordable cost.