The NVG-5600 6-speed does NOT take gear lube. The lubricant specified by DC is Texaco MTX 1874 STF. It is a thin oil on the low side of 30W. It is the ONLY lube you can use and maintain warranty coverage.
I am using Amsoil Series 3000 5W-30 Diesel Engine oil in my 6-speed. It is a proven lubricant in these transmissions but it does place your warranty in jeopardy.
In the diffs, many use Amsoil Series 2000 75W-90 front and rear, myself included.
In the transfer case, use Amsoil ATF. ATF is the specified lube. I am using the 5W-30 engine oil because I believe it offers superior protection. Several are using 75W-90 gear lube. Once again, to maintain warranty coverage, use the ATF. Good luck with your choices. Oo.