Milky appearance usually indicates water in the oil. For tractors parked outside water usually gets into the transmission because the shift lever boot has cracked. It could also be a bad gasket or just condensation. In any case the fluid should be changed and if you can get a look at the insides, especially near the bottom to see if there has been any rust damage.
Does your transmission share fluid with the hydraulic system? If so, that is another source of contaminants.
Do you know for sure that the current oil is the proper product for your application? If not, that is another good reason to change it out.