Hach instruments from Hach Ultra Analytics are workhorses, and among few of the best in the world. However, there are several Hach knockoffs with identical technology but packed in slightly different boxes and under different names, but offered at similar price.
Most particle counter are laser type, and vendors mostly do not recommend them for dirty or black oil like diesel engine oil. Laser wont see the particles when oil is black laser would not penetrate in.
However, there is this process of diluting the oil, making the oil lighter in color so that contaminants can be more visible once laser particle counter is on. Why dont you check this process with your local particle vendor counter.