I had the same issue with my om642 - I filled it up with a measured 8.5 litres and it was well below the max mark on the dipstick - so I just left it, I figured that the spec of the engine was correct, the dipstick was the problem. I repeated that every 10k miles over about 120k in my ownership. It was on the original oil cooler seals at 160k miles when I sold it.