Except even Honest Johns own forum members opinion is that it is snake oil.

The old Volvo 240 forums pretty much agree that it ran on unleaded without any issues, so you got conned.

and I quote

"Volvo issued a leaflet at the time unleaded started to appear at the pumps (early 1990s) which listed all models 200/300 onwards as to whether they can use unleaded or not.
All 200s can run on unleaded, though some need 'tweaking'."

Caused by poor design, repeated running cold for short distances with diesel condensing on cold cylinder walls getting blasted past the rings into the sump + these engines run their DPF cycle about every 200 km for 15 kms. A tiny amount might be due to turning the engine off while the DPF cycle is running, but this would be insignificant compared to the above factors.

All diesels suffer sump oil dilution depending on use/design

Frequent oil changes will be necessary to avoid wear due to running with sump oil diluted with up to 10-20% diesel.

I have had one for 5 years and it is otherwise excellent, so oil is changed twice yearly(about every 6000km)

"This is caused by the driver switching off the engine while the DPF is actively regenerating. " No, this is caused by poor car design - forced by eco-loonary.

Poor design, yes, but also people not buying and using such cars as they were designed to. Both at fault, other than those people who bought early DPF-equipped cars over a decade ago. No excuse since then (on both sides) for people not knowing.