I don't remember the RS800P have any threads to be shown on the axle; unless it is the ribbing on the seal but the seal does needs replacing. The seal is PN GT41857-B.
It may look like you lost a lot grease. First replace the seal and then drive the mower for 15-30 minutes and then feel the bottom of the transmission if it is getting hot then you need to add grease; otherwise, you don't need to. Note the case will normally feel warm just not hot.