1972 VOLVO P1800ES - Gearbox overheating

Hello to anyone who knows why the gearbox would run hot. And the j type overdrive is very hot. cant hold your hand on it...is this normal for gearboxes and od units to run so hot. They have both been overhauld by very good mechanics and has a SAE30 grade engine oil in it.
I dont know anyone else with an ES model so dont have a comparison.
If you have an ES and its running good could you tell me how hot does your gearbox and overdrive unit get after even a short run of 10km 6miles.
thanks very much.
iain