Actually the quoted limit is 1L per 1000km, which is 625miles.
But you shouldn't use anywhere near that amount.
Really? I agree that the book states 1L per 1000Km . . . If I take things real easy and change at about 5k, don't push it hard I might, just might get a litre to last 625miles.
If I 'enjoy' my engine I'll be lucky to see 500 miles from a litre!
But hey, don't panic - it's not 'burning' oil in the old fashioned sense of iffy rings. My understanding is that 10W40 is a peculiar viscosity, in fact not that many manufacturers use it anymore.
As such, to make a usable 10W40 requires a lot of polymers. These polymers have a very low burning point and tend to melt quite quickly; this leads to thermal breakdown in the oil which in turn vaporises in the engine - it does not burn; that takes too long
So, once again it's back to the old trade off, I'm sure VW, Hondas etc can go for 6k miles before needing a top up. However, I'm more than happy to top mine up once a week for the fun of an Alfa.
Btw - 4.5k miles - 6 litres of oil so far and counting . . . it's normal!
Also, why switch to Mobil or any other brand? Selenia 20K is what the T-Spark was designed with - just use that!? The Selenia racing oil is 60W40 and may last a little longer, but Alfa only mandate that viscosity for V6 engines anyway.