You left syn oil in an old 616 for 12k miles without getting the soot load tested? No offense but you probably went over the limit, these engines usually hit the limit around 5k-7k. 12k miles is pushing it for a 603 and probably about right for a highway driven 606.

My 603 would be pushing the soot loading at 7k miles and I do a lot of highway driving. Not many short trips.

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1999 SL500
1969 280SE
2023 Ram 1500
2007 Tiara 3200