Gearbox oil - am I smarter than Kubota engineers?

[Delmar]

I know Kubota recommends their hydraulic fluid for mower gearboxes, but I wonder if heavier oil isn't better. I just can't see any reason to use thinner fluid. There's no tiny tubing or high pressure in the gearbox. It seems like gear oil is the way to go.

Leaking oil seals are the culprit in every case of a Kubota gearbox failing in my experience. Thicker oil would be less likely to leak through the oil seals. At least, that's my opinion.

Anybody else have a different opinion? I know the Kubota engineers make recommendations for a reaosn, I just can't imagine what it is (other than to sell their branded fluid).

 

[Mak65]

Are you referring to a bush hog type rotary cutter? The manual for my LandPride mower calls for 80W 90 gear oil in the gearbox.

 

[Toyboy]

I use 80w90 synthetic in mine

 

[GeoHorn]

I would use what the mfr’r recommends.

From a purely theoretical perspective….thicker (heavier viscosity) oils do not necessarily have lesser leaking tendencies.
In fact, thicker oils resist “wiping” by the seals and at high operating speeds can be forced beyone seals and gaskets.
It’s also easy to “imagine” thicker oils lubricate heavily-loaded gears and bearings. But modern lubricants / additives do not need heavy oils, which also do not easily feed or re-fill into smaller spaces or recoat high-speed surfaces.
Heavier viscosity oils also do not carry heat away as readily.

 

[Hugo Habicht]

I know Kubota recommends their hydraulic fluid for mower gearboxes ....

Kubota specifies SAE90 gear oil for all G1900 mower deck gear boxes.