Question: I had a 1994 and 1995 2500 Cummins Dodge diesel 4WD and changed the oil every 3000 miles using Mobil Delvac 1300 Super 15W-40 diesel-engine oil. The mechanic would always say how clean the drained oil was and that I was wasting my money. I purchased a new 2008 Duramax Chevrolet Truck/Silverado/2500 4WD. It now has 20,000 miles. The first time I checked the oil, it was black. I had it changed, using the same oil l mentioned above. Checked the oil a few days later, and it was black. I went back to the dealer and the techs couldn't explain why. Called several other dealers and no one could give me a reason, some saying not to worry because it's under warranty. I was unable to find a phone number for a GM rep or Duramax. What causes this? Will it cause engine damage? How can it be corrected?

Answer: Two different diesel engines decades apart in diesel technology can be an invalid comparison. Fundamentals tell us the majority of contaminants floating around in any engine's oil are from blow-by -- the exhaust gases that blow by the piston rings during the combustion stroke -- and it's unavoidable. Typically, when there is excessive blow-by, it's accompanied by higher than normal oil consumption. Don't know if they did, but the techs at the dealership can do both engine compression and leak-down tests to determine if you're getting a proper seal between the piston rings and cylinder walls. One thing you can do is monitor the amount of engine oil lost between oil changes. There should be a minimal amount lost in 3000 to 6000 miles. A rule of thumb I use: A quart burned between oil changes is too much -- you should not have to add oil before your next service.

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