I finally decided to try something else in my transmission. Today I drained out the Amsoil Series 2000 75W-90 gear lube that I had installed a year ago. I was never real happy with the way it shifted with this stuff, and things were just getting worse lately, almost next to impossible to shift into first or reverse when at a standstill, especially when the fluid was warm.

I put in 4 quarts of Redline MT90 synthetic manual transmission lubricant, and wow, a night and day difference on how it now shifts! First and reverse selection at a standstill is like butter. Redline is GL-4 rated, for transmissions. Whereas Amsoil is rated GL-5, for differentials. My guess is that the synchros weren't able operate effectively with the Amsoil, and now they're back doing their job.

I know the OEM fluid in the NV4500 is Castrol Syntorq LT, which is also a GL-4 lube. I checked and it's $54 a gallon online, and like $20 a quart from a dealership. I bought the Redline for $7 a quart, definitely a cost effective alternative.

 

You would be right, GL-5 gear oil is not going to help the synchros at all, because they need a different friction coefficient to function well. After experimenting with two vehicles for a long time, I run Redline MTL in an NV5600, along with 8 ounces of Prolong. MTL has less viscosity than MT90. That combo cured the hard shifting and grinding that was common with the DC recommended fluid. There's a lot less metal on my bottom magnet and in my Eaton filter, as well.