The SAE paper basically explains why Kubota was forced to upgrade their UDT specs to Super UDT. Mostly it is due the major advances in their tractor hydraulic equipment over the last ten years or so, especially with hydrostatic transmissions, electronic solenoid actuated gear selection, and the much higher pressures and closer tolerances generated with the new hydraulic pumps. The fact that the hydraulic oil is called upon to perform so many different funtions in a wide variety of different equipment applications is also a major factor for Kubota' new hydraulic fluid specifications (which in Japan and some places in Europe is also biodegradeable as well).
The main concerns Kubota addressed were low temps, viscosity and filtration, and extreme pressure-friction.
Even though two oils are of the same viscosity at various temperatures, this does not insure their filtration properties at the various temps, mostly due to differences in wax formation and water contamination which leads to filter blocking.
The Kubota OEM hydraulic filters have a 35 micron filtration ability (their diesel fuel filters have an 8 micron capacity for comparision). This new hydraulic filtration specification is due to a dangerous situation which can develop prior to the hydraulic oil warming up to proper operational temps before use of hydraulic equipment, ie there is severe backpressure at the filter unit until the oil warms up, thus leading to either no or not enough oil in the hydraulic pump. This condition leads to excessive friction and pump destruction.
Use of Super UDT would be a good bet for HST and GST hydraulic systems, especially in colder climates and/or tractors stored outside where condensation buildup in hydraulic fluid is a major concern, as even a 0.5% contamination with water is enough to seriously degrade hydraulic fluid performance..
