Granted, I do live in Phoenix with low AM temps in the 35-45 deg. area. My NV-5600 does not like going into the lower gears, especially 1 & 2 but, after a short bit of driving it does get better with easier engagement. It, especially, does not like downshifting when cold.
Earlier this year, I tried a boutique gear oil (Specialty Formulations MTL) that I had been reading a lot about on Bob Is The Oil Guy. com. SF's products are considered to be of exceptional high quality using only the best base oils & additives. This product did help the notchy shifting, ever so slightly. Nothing dramatic, at all, though.
I've read of substantial shifting improvements with the use of RedLine, Amsoil & Torco's products.
I'm beginning to think that this transmission, like some folks have said, is a heavy-duty, massively geared unit that was not designed to be smooth shifting rather, an industrial strength transmission that takes time to go from gear to gear.
I have over 40,000 miles on my trans. & it shifts, pretty much, as it did when new. The shift quality has not deteriorated in that time. So, my guess is that it shifts the way it's supposed to.
Joe F. (Buffalo)