"What could cause a substantial reduction in the specific lubricating oil consumption (SLOC) value for a gas engine? There are some instances in which we have seen a sharp decline in the daily top-up of lube oil. What are the probable causes? Is crankcase ventilation pressure the culprit?"
To understand the reduction in oil consumption, you must first determine where the oil is going. This will help diagnose the reason for the measured decline.
Some degree of oil consumption is to be expected in all engines. What is considered normal or acceptable will vary based on the application and the design of the engine.
Some engines consume oil by design from the very first time they are started. The consumption may be as much as one quart per 1,000 miles and yet still be considered acceptable.
Speed and load also affect oil consumption. The higher the revolutions per minute, the more oil will be consumed. The added pressure on the seals and gaskets allows some of the oil to find its way around and get burned away in the combustion chamber.
In addition, the condition of the seals and gaskets should be considered. The older and more worn the seals are, the more oil will be consumed.
High operating temperatures can also impact oil consumption. The hotter the engine runs, the lower the viscosity will be. Once again, it will be easier for a thin oil to reach the combustion chamber and get mixed and burned with the fuel. Keep in mind that there is a point at which the oil’s smaller molecules will evaporate. If the oil being used has a viscosity that is below the recommendation of the engine manufacturer, oil consumption will occur for the same reason.
High volatility is another factor in oil consumption. Usually found with lower quality base oils, the volatility is the evaporation of smaller oil molecules. This oil vapor reaches the crankcase headspace and is often pulled into the intake and consumed by the combustion process.
So what could cause a sharp decline in oil consumption? With so many variables, it can be difficult to pinpoint a single offender. It may be just the right combination of a few factors. The most likely causes are simple things like a viscosity change, a lubricant quality change, a seal that was once leaking but has now stopped or operational severity changes.