What is the maximum safe differential oil temperature?

I have reached as high as 235F while towing a 14,000 lb fifth wheel trailer using the stock GM oil, is this OK or is it too high?

Should I consider installing a Mag-Hytec cover?

Thanks in advance.:)

With a trailer (anywhere from 6000 tounge pull to 12000 5th) on the highway and maghytec cover and fully syn oil mine runs about 100 degrees over outside air temp. I've never hit 200 on mine even before the mag hytec cover.