If it was me I would run it on a decent 5W-40 fully synthetic.
Twin spark engines run very well on it.
However a 10w-40 semi will be fine, and if the engine uses an excessive amount of oil (Many TS engines do) then switching to something like a 10W-50 or 10W-60 can help lower oil consumption.
Back when the 147 was new Alfa had a marketing tie-up with Selenia (Not sure if they still do).
And back then Selenia's top oil was the 10W-60, hence why the manual will likely recommend it for extreme conditions or hard driving.
It does sound like the cam variator needs changing as well, and that is where your diesel noise is coming from.
The cam belt needs to come off to change it, so get that and the tensioners replaced at the same time. The interval is only every 36k or 3 years as they have a nasty habit of snapping.
Giulietta 2.0JTDm (170), BMW 335i Cabrio