Oil is replaced in an engine to remove contaminants introduced in the combustion process and ingested during the sucky cycle. There are no contaminants in a transmission apart from wear materials. I would check the oil level periodically and if it isnt down, indicating a leaking seal or worse, then I wouldnt touch it. Oil does not deteriorate unless external contaminants are introduced. I could see it being of value with a GS or HP where they are pushed hard thru rivers, deep puddles, etc. causing the fluid to cool and contract, drawing moisture into the gearbox. 🙂
