Most Indys use 10W40 and recommend it. 5W40 is more viscose and likes to operate at a higher temp - turbo diesels love it. Indies state that nothing more viscose is needed than 10W40 and they have to be right - I rang 5 indies before having the oil changed on mine in October. I went for 10W40 Valvoline extra life.......after 2 weeks, I went back with Valvoline fully synthetic 5W40 (not the blend, the other one - i forget the name) and paid for another oil change and a new filter so i could successfully change the grade of oil.
On the 10W40 the engine was A LOT noisier on start up and didn't feel anywhere near as smooth as on the 5W40 5W40 all the way.
There is a thread on here in which Alfadoit recommended i go to 5W40 and he gives a nice synopsis (from memory) as to why 5W40 fully synthetic is better...was very glad i took the advice. I don't know if it was me being stupid but it almost seemed as if the engine was tighter when cold and you could "feel" when the engine was warm enough for a damned good thrashing on the 5W40. the 10W40 just made the car noisy and less pleasurable to drive.
Anyway, I have no right to be here anymore - I sold my GT - wish i hadn't...i REALLY REALLY REALLY wish I hadn't. I thought the new motor would continue to engender the same love and attention to detail - it doesn't and now some lucky little git is enjoying my Alfa - bet he ends up making it go bang.