Since I am a fan of Texas Refinery Corporation products, I have poked around their website a bit and discovered it's a very good source of information about different lubricants, such as viscosity and ISO#, intended usages and such. (I am in no way affiliated with TRC, btw.)
Go to their website
http://www.texasrefinery.com/ , and look in "Quality Lubricants". They list:
Cleaners, Solvents, & Degreasers
Fuel Treatment
Greases
Hydraulic Oils & Fluids
Motor Oils
Specialty Oils
Specialty Products
Transmission Fluids & Gear Lubricants
In each category, there is a list of products and each product has a description and (scroll to the very bottom of the page) a link to specifications for that product.
For example, to get information on "303" fluid, select "Special 303" in "Hydraulic Oils & Fluids". There is a good description of that product there plus a nice list of equipment it is designed to work in and fluids it will replace. It appears that THEIR fluid is a direct replacement for Kubota Super UDT. Do all "303" fluids meet Kubota's spec? Maybe...maybe not. You would have to check each brand out individually. As Soundguy says..read the can!