Over the past two months a number of maintenance personnel as well as owner/operators have queried me as to why using a different brand of engine oil can result in an initial increase in oil consumption, even though they may continue to use the correct weight and classification oil.

Changing brands will sometimes result in increased consumption for a certain time period, before leveling off within several oil and filter changes. Keen operators who catch this trend early will often return to their original brand only to find that, once again, higher oil consumption is observed.

Chemical engineers at various oil manufacturers offer the theory that the jump is more than likely caused by the percentage changes that exist in both the detergent/dispersant additives used by different manufacturers. It’s these chemicals that keep sludge, carbon and other deposit precursors suspended in the oil.

Although oils commonly used in heavy-duty, high-speed diesel engines, such as a 15W-40 for example, comply with both the American Society for Testing and Materials (ASTM), and American Petroleum Institute (API) standards, (as well as the Society of Automotive Engineers’ recommendations), there is an acceptable operating window allowing some variation. Oil of the same grade may contain different degrees of similar additive packages – yet still meet mandated standards.

These changes in the detergent/dispersant additives will chemically react with previously deposited material in piston ring land areas, for example, which provided a sealing agent effect.

Remember that engine oil is designed to act as a lubricant between the piston rings and cylinder wall. It is distributed on the pistons’ upstroke by the oil control rings. On the piston downstroke, the oil is scraped from the cylinder wall and returned to the crankcase via the oil drain holes located in behind the oil ring control land of the piston.

When these sealing deposits are chemically removed by the use of a different brand of oil, the new chemistry will eventually produce deposits of its own. Over time they will once again return to their role of sealing agent, which is when oil consumption returns to your historic level. All engines consume some oil during the combustion process – it’s a fact of life. You can’t see any sign of blue exhaust smoke with the naked eye due to the very small amounts of oil consumed, but believe me, it’s in there.

At a level of less than five per cent opacity, no exhaust smoke is visible without using a highly sophisticated smoke meter. Keep in mind, however, internal or external oil leaks – no matter how small – will cause oil consumption levels to surge. So check carefully for drips of oil from gaskets, seals, filters and drain plugs.

In addition, if you are using extended drain intervals ensure that you are following the engine OEM’s recommendations. Waiting too long to change the oil and filters – particularly if excessive light-load operation and idling are part of your normal operating cycle – can result in sludge formation inside the engine.

If oil drain back holes are restricted or plugged, high oil level accumulations can occur underneath the rocker cover. This can result in greater quantities of oil lying in the top side of the cylinder head, which will overload the valve stem seals.

If oil flows down between the valve stem and guide into the combustion chamber it can be burned or become exhausted from the engine. Plugged or sludge-packed oil drain back holes in the piston will also result in additional oil consumption problems.

Most major engine makers now recommend changing the engine oil and filters based on the quantity of diesel fuel consumed, rather than on an accumulated mileage or hours of operation basis.

If you are running a mixed engine fleet in distribution, linehaul or severe-duty service, you need to pay particular attention to the type of oil being used.

North American engines usually feature a “Mexican hat” design of piston crown with higher top rings than do Japanese and European makes. There is a tendency in mixed fleets to use a universal oil in all engines, but this isn’t always a good idea. Many newer high performance engines with common rail fuel systems operate at higher injection pressures and run with tighter operating clearances. It is very important to match the oil to the engine.

Many European and North American engine users are spec’ing semi-synthetic or full synthetic engine oils, although some European and Japanese engine OEMs recommend a super high-performance mineral based oil. Check with your local oil supply rep to ensure that one oil will fit all of your needs.

Choosing the right lubricant brand and viscosity will allow the engine long life with minimum oil consumption.

So the next time you change your brand of oil and notice an increase in oil use, remember this is a normal chemical reaction.

In the May 2001 issue of Truck News there was a slight miscalculation on Page 64.

A 500-truck fleet would actually be able to save as much as 1,324,889 litres of fuel per year; this amounts to an approximate savings of $909,199.90, assuming a fuel price of 60 cents/L. Truck News apologizes for any inconvenience this error may have caused. n