</font><font color="blue" class="small">( I don't understand why brands cannot be mixed. UDT is UDT. What am I missing? )</font>
What you are missing is that different brands have differing chemical properties that might not be compatible with each other. UDT is not a brand, but a specific product of Kubota, as well as Super UDT is also another product. The similarity of names doesn't always mean that it is compatible. Just like the word salt doesn't give you any indication of what type it is. You wouldn't use rock salt in a recipe that called for table salt, even both have the similarity of the term "salt". Whenever mixing or interchanging any product it is incumbent upon the user to make sure that they are compatible by reading the label. Chlorine bleach and sudsy ammonia are both household chemicals, but if you mix them, you will have disastrous results.