When the oil is cold and therefore too viscose the shearing forces in bearings will tear long molecule chains to shorter and destroy the oil. The problem with diesels is that they never warm up at idle if the ambient temperature is cold. You have to put some load on the engine to develop enough heat to increase the temperature.
My manual says the same thing as Kubota.
When I am shutting down the engine I set RPM to about 1300 so when it starts next time there is more lube flow right after start. Then I lower the RPM to idle (1000 rpm) and creep from the garage. I let it warm up for few minutes and increase rpm back to about 1300 - 1500 and creep wherever I need to go. When the temp gauge moves above minimum temperature I increase the RPM and gradually increase the loading until temperature stabilizes on normal operating value (It takes about 10-15 minutes) then I load the engine as needed for the job. Mowing or snow plowing at full PTO speed (2400 rpm), FEL work at somewhat lower rpm to lower noise (2000 to 2200 rpm). Needles to say that my tractor sleeps in heated garage.