Re: GT service - what oil
Alfa dealers that I've used tend to use semi-synth and not always the correct viscosity for your engine, unless advised to use fully synth when they again use any fully synth of random viscosity.
When I got my current car I resolved to always supply oil and brake fluid and have done so! Apart from anything else you avoid getting ripped off.
By my reckoning your car is recommended to take a 10W40 semi or fully synth, a common viscosity (and 5W 40 or 0W 40 would also be fine) so chances are they got it right, except that fully synth would offer more protection for a longer time.
The various engines like mine that are specified to take unusual 10W 60 fully synth (though run fine on equally unusual 5W50) have a much lower chance of being given the right oil, there are examples on here where they've been given all sorts of stuff!