Cheers SonofJoe, thank you for your answer.
My question was somewhat stupid, but I find intriguing all the benefits we could get from "old tech" 20W50 oil, vs more "refined" (pun not intended) thinner, VII loaded oils, like 0W40 or 5W40.
Regarding my car, I found here and there that Fiat was once adamant about its own 10W40 semi-synth oil, and that the semi-synth part was the important part.
I can't seem to find logical reasons to use a 10W40 semi-synth over more "recent" oils, and since I'm a bit bored these days, I just lay here wondering.
Is the 10W part important? How could it be, over a 0W or 5W? Is it the more robust HTHS? Don't think so since recommended oil seem to be very average (Selenia 20K). On engine with worn out cameshaft phase variator (that makes diesel noise at cold start), sure, thicker oil when cold help making this [censored] device shut up, but on perfectly healthy engines?
Is there really a difference between, let's say, a thick "modern" oil like Yacco Galaxie 15W50, and an old school 20W50 like Shell Helix HX3 (I can't seem to find this one locally, 20W50 oils have disappeared from shelves!)? I don't think about the possible longer OCI, better oxidation resistance, etc, but regarding the benefits for the engine. Is it the sulphur in gr I or II?
What about Selenia HPX, that Fiat pushed forward until a few years ago, with miraculous benefits?
Sorry for the off topic, was just digressing.
Edit : that lead us to the most important question, why then are you using 0W20 oil in your car? (I won't take "because it is designed for this type of oils and not 20W50" for an answer, sorry
)