Can I use this oil in my trans?

First off I have a 1978 Jeep CJ-5 and everything is stock at least what I am asking about. When I asked at the shop what to use they said 80W-90 gear oil for the manual transmission and differentials. According to my Haynes I can use this in the differentials but the trans says 75W-90. Can I still use the 80W in my trans? I've heard it's fine to use the same in both. Thanks ahead of time.