1.8 Oil consumption - UK warranty query and has anyone used Wynns Super Charge.

Has anyone used Wynns oil treatments.

As background I bought my 2011 1.8 convertible two months ago with 50k miles on it and it has done 600 miles since. The oil level has dropped from near max to near min in that time. I became aware of oil burning issues on A5s through lots of discussion threads on this on US Audi forums, mostly 2.0 litre. 
It seems over there cars up to 2011 maybe eligible for free piston ring repair . Not sure if that warranty applies here - can anyone advise. I gather an oil consumption test is needed for that.

I  don’t think my use would qualify, some on their forums are using a litre every 300 miles.

I have no external leaks but the exhaust has black soot at the end, so I guess that’s where it’s going. Not seen or smelled burning oil though.

I was thinking of adding Wynns Super Charge high Performance Additive to the oil. Has anyone tried it? 
I’m wary in case adding it negates any warranty. If there is no warranty Wynns say it suits all engine and general feedback has been good, though no one I’ve read mentions Audi 1.8.

Any help appreciated