I asked the same thing, & this was the response.
"We do offer our Diff Life 75w80 if you prefer that
Thicker is not better- in a gear box it can result in foaming and higher temps - actually reducing protection on longer drives."
I decided to go with the thinner MTF10, as that was his recommendation. My Civic is also a slow, gen 7 econo-box with the D17 motor & am not pushing my gears to redline every day. It mainly just sees longer freeway drive times, but RPM's usually stay between 300/400k for long distance & use 5w40 in the crankcase as he recommended to me a year ago where I prefer it thicker anyway. Made sense to me that in a gear box the thinner oil would cool down quicker & flow better, providing better protection, so I decided to give it a go.
