I have a gasoline-engine car that is four and half years old and has been driven for 60 000 kilometers. I have noticed that the fuel efficiency is not what it was when the car was new. However, I lived further away from the workplace and had a longer commute with lower speed limits when the car was new. So, my question is whether an old engine has worse fuel efficiency than a new one, or whether the the change in my fuel efficiency is due to a shorter commute with higher speed limits.
The car has been regularly serviced with oil changes at 15 000 kilometer intervals. The original oil was 0W-20 but I have noticed that in the annual service, 5W-30 oil has been installed. I have noticed that the engine sound when idle might have become somewhat louder than what it was when new, and I can barely hear very slight piston slap after cold start that fortunately becomes soon unhearable after the engine has slightly warmed. I know the engine has special piston coatings but am unsure whether they last for the whole lifetime of the engine.
At least it seems to me that here in Finland where the purchasing tax and annual tax of a car is proportional to its fuel consumption, it is the incentive of the manufacturer to do whatever possible to lower the apparent fuel consumption, even if the solution doesn't last for the whole lifetime of the car. After all, the fuel consumption tests are made for new cars.