<font color="blue"> all it says is J20D and J20C, I dont know what that means. I cant believe it does not give a specific weight. </font>
Garrabo,
I don't know if your manual is different but the on-line manual for the 3120 contains this information:
<font color="blue"> Transmission and Hydraulic Oil
IMPORTANT: Avoid damage! Use recommended oil only. Do not use engine oil or "Type F" automatic transmission fluid.
Choose oil viscosity based on the expected air temperature range during the period between oil changes.
John Deere Low Viscosity HY-GARD transmission/hydraulic fluid is recommended. John Deere HY-GARD transmission/hydraulic fluid may also be used for temperatures above -18° C (0° F).
Other oils may be used if they meet John Deere standard JDM J20C or J20D.
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This is located at the very beginning of the section tiltled Service Transmission. There is also a temperature range chart. There are only 2 oil weights. The standard Hy-Gard or the Low Viscosity Hy-Gard which I think corresponds to the J20C and J20D designations. I only know this because I came across the same thing on Saturday for my 4310. I called the dealer and asked him if he had J20C or J20D oil. His response was "Uh?". He asked me what it was for and then told me Hy-Gard. I looked at the manual again and saw that it was spelled out at the beginning of the section but the MFWD section only states J20C or J20D. No reference is made to Hy-Gard.
The dealer should be responsible for flushing and refilling your MFWD with the proper fluid since he gave you incorrect information.
Jeff