The reason 50w oil was originally recommended by the manufacturer for the NP435 is because gear oil contained sulfur compounds that may damage synchros and brass parts. These are the same sulfur compounds that are used in hypoid gear oil, for hypoid gears (ex: ring and pinion) to prevent galling under extreme pressure. Since the np435 doesn't use hypoid gears, the sulfur compounds served no useful purpose. As far as viscosity, 50w motor oil and 85w-90 gear oil are approximately the same. Now, fast forward 25+ years... Modern gear oil uses low sulfur and additives to combat the ill effects of the sulfur on these older transmissions. So, we can use gear oil in the np435 without any worry. Now that we are on a "level" playing field, which works better?
I know that in 833's, you are supposed to use motor oil for the reasons I mentioned above; but, you can't use synthetic oil because it will not shift properly (not compatable with synchro's and sliders). There are friction modifiers you can add to motor oil to make it work, but most people just use GL4 or GL5, because it works.
So, I guess we can conclude that both 50w and gear oil will work. I have heard that many people find that their 435 shifts smoother, especially in cold temps with the 50w oil. Maybe there is no hard and fast rule on this one, sure is interesting though...
William
