There is a replaceable spin on filter, try changing that. If your tractor has an older system then the filter can be taken apart and washed. It's next to the reservoir on the right side of the tractor.
if the oil is being aireated the fault will probably be on the suction side of the pump or the pump itself, so try tightening all pipe joints leading to the pump and when the engine is stopped wipe all the pipes with a rag to make sure there is not a leak. your oil had to have gone somewhere !