When you see a manufacturer claim that his oil water separator will remove oil and grease to below 200 mg/l or some other number, keep in mind what he really saying. They are usually basing their claim using pure water, at 65 to 70 degrees, with some amount of clean SAE 30 weight motor oil straight from the can. Ask the vendor exactly what they are basing their oil removal claims on. Ask what percent oil was added, ask for exact SAE weight or viscosity and ask the temperature. Bench scale testing of your oily water is usually a good idea to see if a gravity oil water separator is right for you.