My 2004 Honda CR-V AWD uses 5W-20 in the engine and 10W-40 in the transmission. It's all about tolerances anyway. Use whatever the manufacturer recommends.
In viscousity numbers, 50 weight motor oil is nearly the same viscousity as 90 weight gear oil.
The 90 weight gear lube spec is 13.5 to 24 centistokes at 100C. The 50 weight motor oil spec is 16.3 to 21.9 cSt at 100C.
