Changing the worm gear oil used to be in the "consumer service schedule" for the previous model, but for this model it is recommended as "dealer service because it requires special skills and tools". I think I know why.
The filling hole is in the front side of the worm gear housing and it is VERY small. The filling level is a horizontal line, level with the filling hole.
It requires 50ml of oil (or 0,63 of a US qt, or 0,53 of an Imperial qt. I guess this means something to those who use these measurements).
The tricky bit is getting the oil in to the hole, without it bubbling out again.
Recommendation for those trying to do this: Use a very narrow piece of plastic pipe, attached to a syringe (credit to TJJ for the syringe). In this way you can dose an exact amount deep within the housing. Slap on the nuts and washers, torque up to 16Nm and you are done.