Over time the fuel injector nozzles become clogged with carbon deposits, gum and varnish. This means rather than producing a fine high pressure spray which vapourises in the air they inject fuel in the form of a jet which doesn't mix so well. The result is poor combustion leading to reduced performance, increased fuel consumption and poor emmisions. A shot of injector cleaner and a good blast up the road can work wonders for petrol injector nozzles.

However it seems to have little effect on Japanese petrol motors for some reason, possibly because their products are sold globally and often into markets with poor fuel standards.

Fair comment, your experience carries much weight. I know little about diesels, apart from understanding that their finely engineered fuel and turbo systems are susceptible to impurities and cost a fortune to fix. I suppose our elderly 'Yamaha' Focus 1.6 Zetec could be classed as a Japanese petrol motor! Although never had any trouble with V W/Audi petrol engines (so far....), but I've always done careful homework before buying, as it's my, not 'company' money, they're never new and I usually keep them indefinitely, with minimal necessary maintenance by me, well away from grasping garages - apart from the MoT check.