if haynes doesn't recommend changing the oil then there is probably very little point in doing it.
the gearbox oil is not put through the same stresses as engine oil. And quite simply, according to stuff i've been reading about engine oil here and on other websites, engine oil doesn't need to be changed as often either.
i'm pretty sure non of my cars, new or second hand have had the gearbox oil changed as part of a fluid service. i will double check at some point though.