Take an oil sample and have it analyzed. You local Caterpillar dealer can do this for under $10. I use Delvac and change every 7500 miles but I do drive a lot.

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1995 S-350
370K + SOLD
1952 220B Cabriolet
39K kilometers + SOLD
1998 E300D
310K +
2012 E350 BlueTec
120K