I’ve read that this can happen (fuel entering the sump, that is) if the car is performing a DPF (Diesel Articulate Filter) regeneration and the regeneration is interrupted - i.e. the engine is switched off during the regeneration process, so the additional fuel that the regeneration process uses isn’t being burnt off. What types of journeys do you drive? I guessing a high proportion of your journeys may be short when the engine doesn’t have an opportunity to get up to optimum operating temperature, resulting in unburnt soot collecting in the DPF. If so, then you’re likely to be getting quite frequent regenerations and if these are regularly interrupted, that could explain the oil warning light illuminating and the warning message being displayed.
If diesel is entering the sump, then presumably it will be diluting the oil and affecting the oil quality - I’d be concerned what effect that oil / diesel mix in the sump might have on the engine longer term.
I used to drive diesel Golf’s (previous generations / marques; a mk4, 5 and 6) the mk5 and 6 had a DPF and if a regeneration was in progress at the end of my journey, I’d extend the journey to enable it to fully complete the regeneration before switching off the engine so any additional fuel used during the regeneration was fully burned off. The tell tale signs of a DPF regeneration in my last Golf (a mk6 GTD) were;
- increased idle speed
- engine fan running continuously
- a ‘hot’ smell
- increased fuel consumption during the regeneration process because of the extra fuel used to assist the regeneration
- gear change indicator (my car was a 6 speed manual) in the instrumentation display indicating to change down a gear to help keep the engine rpm’s up while the regeneration was taking place.
I’m assuming there will be similar tell tale signs in the mk8.