I would like to draw back to your experience

Since I'm still troubleshooting, I looked at my spark plugs again yesterday. They were all dry so they should work. But they looked like shit. I didn't take any pictures.
Description of candles:
- 2 x insulator torn > immediately replaced
- All. all candles are black and above all hard encrusted with a brown/black layer which hardly goes off.
Is this normal after approx. 25000 Km with candles at the V8 ???
Are Beru Ultra and yes, I know that are not the ones it should be. Oil consumption is normal. Under 1 litre at 1000 km. And still for information I have an LPG gas system in it.
Maybe that's why someone has another tip for me?
What are your experiences?