It is recommended that the fuel filter on a John Deere tractor or other piece of equipment be kept full at all times. A full fuel filter ensures that there is an adequate supply of fuel to the engine, which helps to prevent issues such as hard starting, poor performance, and engine damage. It is also important to keep the fuel filter clean and to replace it when it becomes clogged. A clogged fuel filter can restrict the flow of fuel to the engine, which can cause the engine to stall or not start at all. It is important to follow the manufacturer's recommendations for fuel filter replacement intervals and maintenance procedures, which can be found in the operator's manual of the specific model you have. Additionally, it's always a good idea to consult with a John Deere dealer or a qualified technician if you have any questions or concerns about the fuel system on your tractor or other piece of equipment.