Couldn't be further from the truth.
Check into newer Deere Backhoes,Loaders,Bunchers,Skidders.....heck,the only deere machines that still use hydraulic oil,that i can think of,are the excavators.
If you sincerly believe that is the case why do we have Engine Oil,Hydraulic Oil and Gear Oil.......and not just Oil?
Hydraulic Oils have Friction Modifiers for very small clearances inside Pumps and Add Packs with Elements to keep the Oil Film Stable at Temperature.
Engine Oil has High Detergents to prevent Sludge Build Up, Dispersants to suspend Soot in the Oil and Add Pack's of elements that woud be Foreign/Unnecessary in a Hydraulic Circuit.
Your Argument is not Scientific. You have meerly found an example of a Manufacturer who has engineered their Product to be Durable/Versatile for the benefit/ease of their Customers/Operators.
I dont expect you to put much much value on my opinion but in my work environment we stopped using single Oil in multiple Compartments 20 Years ago. If you put Hydraulic Oil in the Hydralic Compartment (Wet Brakes) on a Large Mining Machine (Read CAT 793) it is un-drivable. They will Shudder/De-Laminate. Oil types have a very specific purpose and just because you have gotten away with it and it works does not make it right. What if running the correct oil allowed the Component to live 40% longer and cost 30% less to Overhaul???