Be careful when changing oil: engine or gearbox ?

In my last oil change, which was actually meant to change the engine oil, I was at the tyre dealer.
I have been there for 20 years now and have always had good experiences.
This time, however, a new employee was at work.
He let off the oil and then replenished.
He was still surprised that there were no 4 or 5 liters.
After about 10 km when I had gone away from the workshop, I realized that my A8 was suddenly driving very slowly.
At home, I controlled the oil and found a fairly high level of oil.
I pumped about 3 liters of oil and then the oil level was about normal.
This caused me to suspect that the tyre dealer's employee simply poured the new oil into the used oil.
But he had drained oil, didn't he?
Now there was not too much oil in circulation, but the driving mode had hardly changed.
He drove slowly, switched unevenly, jerky or partly not at all.
Since I'm thinking maybe you would have dropped the transmission oil instead of the engine oil.
To make it short, it was really like that.
Luckily the mechanic had also not performed the "gear oil change" correctly and there was at least so much oil in the gearbox that it could survive.
I quickly made an appointment at ZF in Pfungstadt near Frankfurt.
After the professional oil change in the gearbox there, the car drove again as if it had got a new engine.
It switched fast and soft again.