Originally Posted by autosafari
Could you please explain this one in detail.
There is a number of reasons for it.
When the engine isn't driven hard at times the build up of carbon deposits, tars and gums is soaking up fuel that can't be burned properly. This is when governments think that unleaded fuels are so good for the environment.
Additionally it will encourage sticky piston rings Sticky piston rings will eventually hit all engines to a bigger or lesser degree. This will occur from 600kms on already when driven extreme short distances or the car poodling along.
There are a number of other effects resulting, but this would go too far.
To combat this every now and again, take the car for a good B-road blast and before servicing get a very high quality engine flush and drive with it for as much as a tank load of fuel would last (Don't do it with any flush as some will not be suitable for it!).