Probably an AAMCO put in a crappy fluid or some do it all shop. Just change and keep on driving.
Enjoy the ride. The 606 is an unbelievable ride and I like it a lot. The engines are tanks and so are the cars. Amazing torque, NA or turbo. Just keep up with it and it will treat you well. Sorry to hear about the previous E300. May it rest in car heaven.
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Only diesels in this driveway.
2005 E320 CDI 243k Black/Black
2008 Chevy 3500HD Duramax 340k
2004 Chevy 2500HD Duramax 220k