I've had some cars which seem to drink oil and others which never seem to consume any.
In my A3 I have to top up with 1 litre of oil between it's annual services which I don't think is all that bad really.
Is it the 16 valve engines that are more oil consuming? Do you think it depends more on mileage?
I think your A3 1.8T is a 4 cylinder, 20 valve motor - 3 inlet and 2 exhaust valves per cylinder.
A litre of oil between annual servicing is no cause for concern at all. I am assuming that you cover 12,000-20,000 miles per year. Please tell me if I am wrong with the assumed mileage.
It's not necessary to keep the oil on the MAX mark on the oil-check dipstick.
More importantly, don't let it go below the MIN level or over fill it to the point that it's over MAX level when checked cold with the car on a level parking surface.
VW Engines have, for years, used engine oil to wash away carbon deposits and other crap that could otherwise cause problems.
Your's is turbocharged; turbos do emit a fine oil mist into the intake manifold; it's a byproduct of how turbos work, and an unavoidable one.
My Pug 406 (the now destroyed one, about which I'm still not thrilled) used to require a bit of oil here and there between services. I suspect that this is because of the diesel engine's very BMEP (brake mean effective pressure).
If I were the owner/operator of your Audi A3 I wouldn't worry at all about a litre of top up oil between services.
What I do suggest is that you stick to the exactly same oil throughout.
Don't mix at all, whether or not grade. viscosity index etc match.
If you drain (perhaps flush - not sure that it's necessary) and fill with Millers XFE then top up with XFE.
If you go for AMSOIL's euro 5w/30 then top up with it.
Don't get carried away with the top up thing. Don't bother unless it's under halfway between max and min. even if it's under half, bear in mind that under 1/4 litre is enough to bring it above halfway.
Overfilling can wreck engines.
Oil peeeing oil into the transaxle etc.