Why does a diesel engine become weaker over the years?

Hello!

I suspect that the engine in my '96 Golf 3 TDI with the 1Z engine has become slightly weaker. Not much weaker, but a little bit. I feel like the engine ran smoothly around 120,000 km, and now at 210,000 km it's not quite the same. I might be wrong, but have you noticed anything similar?
I've already replaced everything new. Air filter, diesel filter, etc.
The decline in performance was gradual and not sudden. Therefore, no one who drives my car would notice it.
By the way, the turbo is 100% okay.
I also believe that the diesel pump needs lubrication, so I add 1 liter of 10W40 oil to my diesel tank every 20,000 km. However, I suspect that the pump may have some wear and tear, as it is a mechanical component. Even with good maintenance, this can happen. But I don't think there's a defect.


Maybe someone could tell me something about that!

Thank you in advance.




Best regards,
Andres Ortmann

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