A few choices... Keep the original mineral oil refrigeration oil and run a b*u*s*t*a*r*d refrigerant like Freeze 12, or get all the mineral oil you can out of the system and install POE oil and R-134A, or COMPLETELY flush the system of mineral oil, replace the compressor and dryer, and install PAG oil and R-134A. The BEST and Deere-approved method is the latter.
DO NOT mix the original mineral oil and PAG oil. (Can you say "Jello"?)
...Bob, refrigeration tech, certified in mobile and stationary systems.