Why not. Oil is there to keep metal off metal. The harder (better) the film the better the protection. In things that have to pump oil to get initial lube, thin oil is necessary in winter conditions but has to thicken up as temps rise. Otherwise the film isn't firm enough to prevent metal to metal contact.
First of all, all the LU/mechanical gear box lube that I have ever seen doesn't recommend "not" using their lube under some conditions. All the bla bla is about how much better it protects.
Your LU is full of lube. Nothing has to be pumped in cold weather as it's continually submerged. Thicker oils (and not that much thicker) provide a better film.
I've been running equipment requiring lube since 1955 and never lost a gear case....well one... a bat wing 3 blade shredder that had 40 hp gearboxes being driven by a 100 HP tractor got one of the sections high centered once on a terrace and it took out the gearbox......the gear box design is normally heavier than the drive mechanism for obvious reasons. The designer of this mower apparently didn't take into consideration that if you high center one of the sections you essentially have a 40 HP gearbox trying to stop the shaft from turning being driven by a 100 HP source.
Another case in point: I bought a new 65 HP 4wd tractor in 2007 that recommended using the same "hydraulic/transmission/differental" fluid in the front axle gearbox that was used in the sections listed.....that's fluid, not gear oil lube.
Under heavy stress one day I took out the front wheel bearings. I replaced the broken parts and filled the gearbox with 90w-140 gear oil. That was over a decade ago and no front end failures.
Fast forward: Buy a new 25 HP tractor, 4 wd, same brand in 2021. Owner's manual and embossed in the fill plug for the front axle is 90w meaning that they wanted gear oil, not "hydraulic fluid" protecting the front axle gears.....did they learn something????? Of course they did. But when I got it home I drained out the 90 and installed 140. Both tractors have front end loaders which can put a lot of stress on gear boxes.
So sir, if it were my machine, you see what I would do......go for it!
