Bela ,
I have had no experience with a J type overdrive but most overdrives used in other cars have a common gearbox and overdrive oil of about 30 grade .
I would not use EP90 as it has additives which could make the clutches in the overdrive slip .
I suggest you try a Gearbox oil of 30 grade without friction additives .
"Penrite" has these possibly in different grades .
If you can not locate any gear oil , try a cheap Mineral 10W/30 engine oil.
But my suggestion is not a recommendation as I have not used a J type.
Surely others on this Forum are using the J type overdrives and can advise!!!
Geoff
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