From my own experience, I recommend changing the transmission oil once a year. Long journeys in the rain in particular lead to water penetrating the engine housing, which can lead to corrosion. In my case, the drive of the rev counter was rusted after the motorbike had not been used for about 1-2 months after a long ride in the rain. The oil level was significantly higher than normal because the oil had mixed with water.
The original rubber seals are now over 30 years old, the replicas (depending on the supplier) are not always durable and permanently tight. So what was not an issue 40 years ago can be a problem today

Best regards,
/Christian