Using synthetic oil(Mobil 1), in my '86 300SDL, changing the oil filter every 5,000 miles how long can I go without changing the oil. With diesels does the soot builup in the oil become a factor in determining oil change intervals?

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jcciem
'94 SL5000 60,000
'91 350SDL 545,000
'91 350SD 197,000
'00 E55 AMG 77,000