What the owner manual says is...........
For diesels >>
"Synthetic-based oil, grade SAE 5W-40, FIAT 9.55535-N2 qualification"
Original fluid or lubricant - Selenia WR Contractual Technical Reference no F 515.D06.
"Synthetic-based oil, grade SAE 5W-40, FIAT 9.55535-H2 qualification"
Original fluid or lubricant - Selenia StAR Contractual Technical Reference no F 216.D05.
"In diesel engines, in emergency cases where genuine products are not available, lubricants with min. perfomance ACEA C2 are accepted. If this is the case, the best engine performance is not guaranteed. We however recommend replacing the lubricant with those recommended by Alfa Romeo Authorized Services.
Use of products with low-quality properties than ACEA A3 and ACEA A4 could cause damages to the engine that are not covered by the warranty"
"For very cold climate conditions, ask to contact Alfa Romeo Authorized Services for the appropriate Selenia product to use"