My bolt has oil for the reduction gear but it’s not part of the regular maintenance. Expected interval for that would probably be similar to a transmission fluid change.
It’s probably built so that the fluid lasts for the lifetime of the car, or whenever there’s any issues it’s able to be changed. Only thing is lifetime is different for everyone especially depending on driving conditions.
“Lifetime of the car” means different things to manufacturers vs drivers. VW would have you believe that their transmissions are maintenace free and will not need to be serviced for the life of the car. They neglect to mention that they only expect the car to do 100,000km. They’re not exactly wrong, in that when the transmission starts to fail you’re probably going to write it off rather than drop 5k on a replacement valve body, but if you had serviced it every 60,000km or so, there’s no reason that transmission shouldn’t be good for 300,000.
