If your motor oil is HD SYN 3 then yes you can use that in the transmission......it’s what my dealer puts in routinely. 3 stands for all 3 holes......
Just because you can doesn't mean should, has anyone ever asked themselves what the difference is between say a 2016 CVO 6 speed box and a 2012 FL 6 speed box with regard to lubrication? I'll save you the trouble, nothing is the answer, yet HD recommend different oils for each, one considerably dearer than the other, both oils will get you through warranty periods, even extended ones, but one is far better suited to a gearbox than the other.
He didn’t ask if he should. He asked if he can, at a pinch. The answer is yes.
Can do what he likes, just pointing out what HD does with regards to trans oil.
Yeah.....so am I!