I always use the recommended 0W-40 Mobil 1
I believe the idea behind 0W vs. 5 or 10W is to protect the turbo on startup and the 40 vs. 30 is to protect the turbo and the engine at higher operating temps.
I would not recommend trying to save a few dollars by using lesser oil. It isn't worth it.
Good catch, he'll have to read the manual to get the recommended oil. I have no idea about dieselscurrently using Mobil 1 10W40, prior to that Pennzoil 5W30, prior to that M1 0W40... originally 0W30 Saab Turbo Oil until it was killed by GM and the Mobil 1 Product placement.
The current and revised 2.0t/T spec is 5W30 in the Dexos Flavor....
IMO your safe with 0,5 or 10 in the 30 or 40 range.... opinions will vary, go read BITOG, nothing like an Oil Thread
Edit: Ah... just noticed the OP is a diesel......