Thanks for the replies.
I'm under the impression that detergents are not the same as dispersants.
Detergents clean surfaces, dispersants are what keep particles in suspension.
It may be that detergents have some dispersant action? I don't know.
Sometimes it seems that multi-grade oils are assumed to be high detergent, but I don't see a direct connection.
It could be that a low- or non-detergent oil contains viscosity modifiers?
To try to answer the query about what's so bad about having particles in suspension -
When a full flow filter is fitted in line before reaching the bearing surfaces, the particles are removed from the system in time.
With no full flow filtering, the particles could theoretically give the oil some abrasive character.
Better to have them fall away to the bottom?
No doubt Lister engines can last a very long time with any oil.
But I wonder if they'd last a very, very long time with a particular oil.