There are oil in the motor. We know that oil will wear down.
Why doesn’t Tesla recomend any kind of oil service with this motor?
An internal combustion engine needs to deal with contaminants (combustion products) and very high heat (we push the limits of heat tolerance to get more efficient engines). There are many additives to help deal with these nasty conditions for the oil, but eventually they're depleted or denatured and thus require replacement.
The heat load in an electric motor is less "spiky" (more consistent) but can also be fairly hot, which is why they're liquid cooled as well. However, there are no combustion products that contaminate the oil. In an electric motor with permanent magnets (Model 3 rear motor), cooling is especially important so as to not demagnetise the magnets.
I suspect they've determined the heat load over time does not denature the oil sufficiently to require replacement. On the other hand, perhaps there are other issues (gear failure, failure of the motor windings/short, inverter failure) they expect to occur before the oil fails, requiring bulk replacement of the drive unit.
For comparison, some manufacturers (and different applications aside from consumer vehicles) have sealed automatic transmissions that don't necessarily require replacement of the lubricant.
There are no magnets in the M3 motors./QUOTE]
One of the surprises when M3 was launched was that Tesla had abandoned the induction motors used in MS and MX in favor of a permanent magnet motor. The induction motor had been one of the biggest differences between Tesla's power train and the rest of the industry.
The change improved efficiency somewhat and, I think, reduced weight, but removed the capability of the D versions to load shift from rear motor to front (at a higher gear ratio) for improved Wh/mile efficiency at freeway speeds.
There are no magnets in the M3 motors.
The oil seen in the video in question is in the gear box - not the motor. Tesla does recommend replacement of that oil every 3 years.
One can find pictures of the interior of the model 3 motor on the web. It's not that exciting being that it is pretty much a garden variety three phase induction motor. The squirrel cage is so tightly integrated with the rotor stampings that the rotor appears to be one big homogenouus cylinder. The stator of a three phase motor is not that interesting unless you are trying to figure out things like winding pitch.
What would be interesting to see would be how the rotor is cooled.