What can be said about the 4-speed New Process NV833 overdrive manual transmission?

[Gregski]

Originally Posted by hatzie

The MM7 is the 4x4 version.
The MY6 is the 2WD unit you just bought
.
It's pretty much the same aluminum case NP A833 Dodge used in the Ram trucks with a GM input shaft and output spline count.
Make sure the countershaft plug seals properly and the countershaft isn't rotating in the case.. And don't use gear oil in it...

Thank you, but what do you mean "don't use gear oil in it" than what should I use and why?