[LouNY]

I would not recommend any such substitution. There is a considerable difference in "tractor transmission hydraulic " fluid and automotive ATF.
The JD J20D is a high quality para-synthetic fluid. Any quality para-synthetic tractor transmission hydraulic fluid will be a good choice, either Deere or CIH, NH or quality after maket.

 

[LouNY]

CNH and NH are not matches for J20C. ATF+4 is a good match. It's full synthetic and works very well with manual transmissions, transfer cases and power steering units.
ATF+4 also the chosen to match Mitsubishi applications where an MS-9602 type fluid is specified. The same fluid type in many Mitsubishi tractors in Japan. J20C is really the Yanmar TF500A. This all goes back the Japan tractors and automotive vehicles from the 70s and 80s.

Who cares about J20C, per the OP;

Anybody know if I can use tranmission oil from supercheap auto for my john deere, please? It recommends J20D. I

It recommends J20D.

You may be a YANMAR GURU but I have seen IH Hytran used in many large JD's with no issues and I"ve seen JD Hygard used in IH's with no issues.

I would have to see a good technical article comparing "Hytrans" to ATF or "ATF+4" and in many years of tractors I've never seen such an article.