I have tested synthetic oil in my 240D and have found that 10,000 miles is the limit. The soot level at that point was close to 5%. The new diesel oils can handle 6% without a problem but the viscosity was starting to go up just a little at 5%.
The TBN of the oil was still at 12 which means it was almost like new in terms of neutralizing acid. Wear rate remained the same at the 5000 mile and 10000 mile check and that is what is important. It had a very low chrome (1ppm)and iron (33ppm)
The oil that I tested was Petro Canada 5W40
I am now using Pennzoil Long Life because it is cheap and meets all the important requirements. I will not be driving the car in the winter so there is no real need for the synthetic oil. I will have it tested at 5000 miles.
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