Originally Posted by Steve
If water under very low pressure can leak in at the prop shaft it seems oil under pressure would leak or gush out. Is there an oil sheen after running the thruster?
The unit most likely uses lip seals which use the pressure of the fluid to aid in pressing the seal against the shaft. When the pressure on the inside of the housing is less than outside, fluid may flow to the inside if the seal or shaft is worn.
High quality units use a double lip seal to help prevent this from occurring. If the external seal uses a stainless steel garter spring that has corroded away allowing the seal to relax its contact on the shaft, the chance of leakage from outside in is increased.
Have the oil tested and see if there is a high sodium level ... if so then the seals are leaking or something is spraying seawater into the hydraulic tank vent.