Narrowboat gear box oil change- will this repair the problem?

On the previous video , My friend went past on his narrowboat, only for the reverse not to engage, will changing the gearbox oil resolve the issue ?

[@TheKurtis99]

Great content Dan 🏆

[@delticman]

Thanks kurtis , hope your doing well

[@user-ht1yp9qn6e]

I seem to have this issue but it engages perfectly in reverse but struggles in forward, would this be the same issue? ATF needs changing? I have checked the oil and it seems very black

[@BullyBonanza]

What gearbox was it?