Mixing two different oil viscosities

Absolutely fine .

My Audi dealer told me that mixing those two viscosities isn't an issue as well

I'm assuming the long life 5W30 is in the engine because you are on variable servicing (rather than fixed interval)? The long life oil has additional additives to prolong the life of the oil. If you were topping up frequently with 0W30 non long life oil then potentially you could end up with a situation where most of the oil in the engine was not long life oil and that could cause issues with extended service intervals. There are sensors that monitor the oil quality -when they detect the oil is dirty or degraded the car tells you it needs a service. So it's not about the minor difference in viscosity but the fact that one oil is long life and the other is not.

I found this in a VW document on service intervals:

"Do I have to use LongLife oil all the time?

Yes. If you want to take advantage of the LongLife service regime. Please note that if the engine is topped up with more than 1⁄2 litre of non-LongLife oil between services, the service indicator will come on earlier and the time or distance between services will be reduced. "

On the other hand, if you are on fixed interval it won't make any difference.