0-10 degrees ! If only for a day or two may not be a problem but if you have those temps for extended periods I am not sure if the 75-90 is a good choice.
How about switching to a synthetic of equivalent rating ?
Better in the cold for both the trans and the arm.
Also, like was said do you wheel this in water, if so start checking seals and gaskets.
Just thinking back when I had a Mustang and I rebuilt the trans, I thought I did a great job and basically I did but I had forgotten to torque the front seal retainer bolts which also held in the main shaft; the bolts loosened up after a few power shifts and didn’t allow the shift forks to function properly.
So thinking on this same lines, did you do any work on the trans lately, maybe a loose mounting bolt, maybe a rear mount not tight (misalignment).
Again, i am just throwing out ideas.
Do it right the first time !