Did a search but found nothing much, so sincere apologies if this is 'old ground'.
Engine oil, pre change flush chemicals such as, https://products.liqui-moly.com/engine-flush-plus-3.html
, for example. They use words and phrases like 'removes troublesome deposits', 'sludge' and 'lacquers'. Words to frighten the layperson.
Are they a useful 'tool' that will help keep your engine in tip top condition?
Total waste of money and of no benefit in a correctly maintained engine, one that has oil changes done at the recommended intervals, using quality Oil of the correct specification.
My unqualified take on this is as follows. If an engine has 'troublesome deposits', 'sludge' and 'lacquers' then your Oil change regime is wrong. Use better oil and change more frequently.
Difference maybe in Gas or Diesel engines?
That may well be a gross over simplification so I would appreciate some opinions Gents please