Why Does Diesel Motor Oil Turn Black? – It is normal for diesel motor oil to turn black quickly. The dark color is a sign that the oil is performing correctly by keeping byproducts of the combustion process in suspension.
How It Works
The high pressures in the combustion chamber forces soot and other contaminants into the oil, which carry them to the oil filter. The contaminants are what turn the oil black, according to Quaker State.