Hello chaps

I know this is probably an old question but, i'm currently trying to fill the gearbox oil, i've been told that the gearbox oil needed is specialist and this is even stated on the GGR website but I went to Ford the other day and asked for this oil and he gave me 75W90? Is this correct?

According to the GGR website 75W90 is for the diffs.

Thanks in advance

Paul