Fair enough.
First place I would try is directly from Beneteau.
Contact Beneteau directly — For vintage/heritage models like the Oceanis 390 (1986–1991 era), reach out to Beneteau customer service, parts, or your local authorized dealer. Provide your boat’s Hull Identification Number (HIN) — it starts with “BEY” and is usually stamped on the transom or in the cockpit. They sometimes supply digital copies or guidance for older boats.
They may be able to provide you a manual in Italian.
www.beneteau.com
I assume that you are seeking general knowledge, not trying to solve a specific problem.
One thing about sailboats that is not well understood by new owners, these boats are made up of individual systems often provided by separate companies. The builder assembled the parts in their hull.
I note that the general specifications for your boat indicates the diesel auxiliary is a Perkins 4.108. It is the younger sister to the 4.107 I have in my boat. Solutions to engine issues can best be sought in the SBO Engine & Propulsion forum or using the Perkins shop manual.