While the X3xx and X5xx share the same frame the X3xx is a lawn tractor while the X5xx is a garden tractor.

To answer your specific question, the X500 has a Tuff-Torq K72 transaxle which has a diff lock and you can change the oil where the K4x is not serviceable so you can't.

Here is a transaxle comparison for you...

Bear in mind that JD sells many thousands of the X3xx series and has for years and provides a long warranty on them. Those few who post about transmission problems are a very small percentage of X3xx owners and when pressed often admit that they are using their lawn tractors to go a garden tractor's job so it's no big surprise that they had transmission problems.

So the question you need to answer is... how much property do you have, what is the terrian like, and what do you want your new toy to do?

X300 = K46

X320 = K58

X500 = K72

Every complaint I've read about the transmission in an X3xx has ALWAYS been the K46 in the X300.

I can't recall reading a single complaint about the K58 in an X320.

You might want to review your research and check that.

The K-72 is a great tranny and easy to service . I use the wheel lock feature every time I use it.