I use synthetic GL-4 rated Royal Purple 80W-90. It can be found at any NAPA store. I'm not going to say it is the KING of oils but it has worked well for me. I tow 10K on a regular basis. Contrary to popular belief, I believe there are many different oils that will work in the NV4500 better then the stock stuff, but that is just my opinion.
I called many transmission rebuild places, including the infamous Standard Transmission in FW, and not a single one could tell me why you have to use the Syntorq or what would happen if I didn't. I had to ask myself:
1. Why did my transmission go out in the first place with the stock oil (I had lower bearing failures at 90K, synchros were still good)... I believe this to be oil related since they are ALWAYS in a bath of oil.
2. What makes this transmission any different from other trannies that require it to use this special oil. Nobody I called could answer that one.
In the end, although lots of people have had good luck with the stock oil, I FEEL <---- my opinion, the stock oil does not provide ME with enough protection to keep my bearings from burning up. So, I wanted to try a different oil, and I prefer an oil that I could afford to change on a regular basis.
My main belief on the NV4500 is this: I feel that changing the oil (USING YOUR BRAND OF CHOICE) on a regular basis, say every 25k, is the key to having a long lasting transmission. My 100k rated oil at 90k looked like s__t and I had two bad bearings to back this up.