What are best practices for the NV5600 maitenance?
The manual would leave you to believe the fluid should normally not need changing.
I have read a few threads about people adding an extra quart or filling the tranny completey.

Also, I believe Texaco made the only authorized syn lube for it, is there a good reasons for that? Common sense would say yes. Comments?

Any advice from factory service people or shop owners who specialize in this?
I have read a lot of threads and it would seem the 5600 is a stout, reliable tranny.
I only have 30k on my truck,the tranny drives like new. I suspect there a lot of happy 5600 owners out there.

 

Redline MTL seems to be a good substitute. Synchomesh from Pennzoil available at autozone also seems to be great and it is approved for the NV5600.

I would change it at your mileage and overfill by 1 quart and add the tranny filter (pto cover kit) from www.genosgarage.com.

That should keep your tranny going strong.