Why would the guys at O Reilly's know anything about what your NV5600 should take?

I would only use the following:
Pennzoil Syncromesh(Autozone), Redline MTL (www.synlubes.com), Or Amsoil MTF.

 

I'll only run Amsoil in my truck...
Scott

 

If you want RP use the Synchromax. But definitely get the 70-140 out. I would also try to find some way to flush out any residual 70-140 so as to not contaminate the fresh oil. I have heard that the Redline MTL is also good and some people report better results with one over the other. As for the overfilling, I had oil on the bottom of my bell housing and it looked purple when I checked it. I drained a little of the oil out and then the leakage stopped.