Waste Steam Emissions
When the engine consumes oil, due primarily to contaminant-induced wear, oil enters the combustion chamber, burns with the fuel, and is pushed out with exhaust gases as particles and volatile hydrocarbons. New mineral-based lubricants have a more volatile light-end fraction and are more prone to hydrocarbon emissions. The level of exhaust emissions increases considerably over time corresponding to engine wear and deposit formation in the combustion zone. This leads not only to greater concentration of exhaust particulates, but also to a higher percentage that are unburned hydrocarbon, a by-product of oil consumption.
Unlike a new engine, the lubricating oil is a dominant contributor to particulate matter (PM) emissions in aged engines. The obvious strategy to control / reduce hydrocarbon emissions is to reduce oil consumption. This, of course, points to a strategy of reducing abrasion and wear. According to projections by Barris of Donaldson Co. after 12,000 hours of service, an off-road diesel engine can produce nearly six times more exhaust emissions due to wear associated with particles and other causes.
Reference: Jim Fitch 'Clean Oil Reduces Engine Fuel Consumption'