Never too late for synthetics. Once switched an 84 325e to Mobil 1 at 200k miles with no problems. My C280 also runs Mobil 1 with no problems or leaks.

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Ali Al-Chalabi

2001 CLK55
1999 Dodge Ram 2500 Cummins Diesel
2002 Harley-Davidson Fatboy
Merlin Extralight w/ Campy Record