Grading system is different so 75w140 gear oil would actually be pretty close in viscosity to a 10w60 engine oil. Slightly thicker. Not recommending it, just letting you know that isn’t as far off as it seems!
I really can’t imagine any reason to go above 5w50 based on what I’ve seen. When it comes to preventing internal parts from touching each other, an inch is as good as a mile. If things aren’t wearing with an 5w50, a 10w60 obviously can’t improve on that in any operating conditions we would ever see. The factory 0w40, on the other hand, did leave us some room for improvement. The wear rates never lie.