Prolly designed to be driven in water so sealed with no vent. Who knows ?? But it does need some venting to stop warmed oil building up pressure from being forced out through the seals. Also probably not a problem unless working long periods of time on a warm/hot day.
Search should include the model numbers 4200, 4300, and 4400. Same axle likely, but procedure likely the same.
for example:
https://www.tractorbynet.com/forums/threads/replacing-mfwd-spindle-seal.312633/
Welcome to TBN
Search is your friend. There have been threads on TBN that address the task you are asking about.
Similar.. are you saying you found yours with gear oil as well?