If you look in the owners manual, you WILL find there are a few different oil viscosities that you can use. My car drinks 5w30 when driven hard. Give it 5w40 and it barely uses any oil over 1000 miles. And both are VW approved oil's for my engine. So the RS6 will be the same, it will be in the owners manual. So as long as its a VW approved oil, with the correct codes on the bottle as stated in the cars owners manual, it will be fine, and there will be no warranty complaints.
Its not like I said to go from 0w30 to 20w60 for a diesel Toyota was it.
This guy has a 2020 RS6 from new. I have a 2006 2.0TFSI from new, from day 1 its drank 5w30. Its like water. Adding Forte oil Fortifer stopped it. Using an oil like 5w40 also had the same result. 109000 miles on it today. Been remapped for the last 60k, run's like a dream and still has 3/4 oil left if I left it the full 10k between oil change.
Hope you get it sorted Lee, I've had my eye on an RS6 for a while.