When filling new engine oil on Monday, I noticed that the old Mobil 1 5W50 oil was contaminated with deposits (many small black hard particles). MY98 80.000km frequently used on trackdays. What is causing this? Oil overheating in the turbo? I change oil every 5000-7000 km, but sometimes this includes 2 trackdays. Should I change oil after EVERY trackday? Each "heat" lasts ~15 minutes, is that too long? I don't have an oiltemp sensor, but the water temp never raises above normal.

 

Most likely to be old oil left from oil changes that is breaking up due to the extra heat of the engine on track days.