I use the 80/90 gear oil because I had a small Kubota B26## that had a very very small leak from front end. Dealer said common and just switch to 80/90 and it would probably go away. (huh I thought) so I did and he was right. 5 years on no more leaks. Dam I love living within reach of a good dealer.
Me thinks its not a critical decision as the viscosity difference is like almost 45% different yet they recommend both??
Yes, it is confusing about the front axle 4wd lubrication recommendation. There is some history and even a bit of reason behind it. It started when Yanmar invented and patented the bevel gear type drven front axle that you see on almost all utility size tractors today. The big advantage of their new front axle was it enabled full power to the front wheels at any steering angle. That was a huge advantage because the old constant velocity joints in use in everything from WWII jeeps to big Ag tractors had a problem with handling full power at tight steering angles.
I bought a litte Yanmar in 1980 - mainly because of being fascinated by the engineering in such a little tractor. And of course I immediately took it apart to have a look.
Yanmar initially recommended 90 wt gear oil - there were no multiweight gear oils on the market at the time. By the next decade (1990s) the Yanmar type axle was in wide use, and there had also been some changes in oils. Multiweight gear oils like 75/80w-90 were now available... And there were lighter viscosity trans/hydraulic oils on the market that now had additives to handle bevel gear type of shearing forces. In years before, many of those trans/hydaulic oils had additives that were not compatible with some bushing and syncro materials.
So about 1990 it became possible for a trans/hydraulic oil to handle bevel gear loads with additives. The reduction in wear was similar to how the old thicker gear lubes had handled the bevel gear loads through viscosity. At the same time, design engineers began to look at lubrication differently.
That is why we see see two recommendations for front axle bevel gear systems: It is lubrication philosophy. One design group prefers a thin oil with additives, and another philosophy prefers the traditional more viscous gear lube. The lubrication shearing forces are the same, but solved in different ways.
Not surprisingly, the lighter UDT type trans/hydraulic oil flows better especially in the cold ..... but the heavier lube leaks less past the seals.
I run 75w-90 in mine, YMMV.
enjoy,
rScotty
