Not stupid at all, Dave.
There was a big problem with Ducato and “engine oil” a few years ago, I think. As I remember it, Ducato’s original oil recommendation was 10/40. They changed this later, to a a 0/30 recommendation (not sure why). But… the Law of Unintended Consequences took over and this thinner viscosity oil led to oil seeping from the crankshaft – not a serious problem but not desirable either! Fiat have gone back to a 10/40 recommendation.
This is what I’ve picked up from various conversations/forums so may not be 100% correct.
As you say, there’s an awful lot of conflicting opinion on the InterwideWebWorld-thingy. I use any good quality 10/40 (suitable for diesel use) in my turbo’d 2.8. These are robust commercial engines so I shouldn’t worry too much, Dave.
Here’s a link that has some good information (Fiat Forum) https://tinyurl.com/y4v3jb4t
Let us know if you discover any more on this topic though.
Barry