hope the gearbox box does not need any oil, but if it does need to be topped off.
you never go wrong in using john deere or case ih corn head grease.
it is a great lube for all most all gearboxes and even thought it comes in a tube like grease it is a much thinner type lube material.
one of the best lubes for gearboxes out there, many use it in old bushhog gearboxes or hay rake gearboxes that leak, the cornhead grease will lube the gears and not run out like gear oil.
check it out.