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We've supplied quality 4WD accessories to customers from Iceland, Japan, the USA, Europe, Arabia, Africa and South America since 1996.

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163 Abbotsford Road, Bowen Hills,
Queensland 4006 Australia.
Phone: +61-(0)7-3252-4039
Fax: +61-(0)7-3852-1808
Email: info@motorcare.com.au
Our terrestrial opening hours are
Monday to Friday 8am to 5pm.

Oil Thickening in Light Duty Diesel Engines


From time to time we encounter instances of excessive oil thickening in light duty diesel engines. While the factors that contribute to this are many and varied, the obvious culprit in the minds of most motorists is the oil itself, which is, in reality, the least likely cause of the condition.

High performance engine oil marketed by major reputable oil companies has been developed to the point that in heavy duty diesel highway engines, 40,000kms between service intervals is not unusual and the oil is still in reasonably good condition. The exact same lubricant, usually a SAE 15W/40 meeting at least API CF4/SG specification, may not go to 5,000kms in light duty engines without significant thickening and deterioration.

The major difference in oil performance is due not only to differences in engine design, but mostly due to operating condition. A diesel engine does not achieve combustion efficiency until it reaches operating temperature, which usually takes around 15 to 20 minutes of running. During this warm up period, incomplete combustion deposits excessive quantities of carbon and soot on the cylinder walls where it is collected and absorbed by the dispersant additive in the oil. Frequent stops and starts also increase the moisture contamination of the engine.

As most light duty engines are not spectacular performers, most people tend to drive them hard through the gears, which leads to overfuelling and further contamination of the oil. The combustion by products from petrol engines are volatile and can be driven off once the engine has run at full operating temperature for a period of time. However, the combustion by products from a diesel engine cannot be driven off by the engines temperature, and as such are totally accumulative.

The build up of combustion residue eventually leads to the oil becoming very thick at cold start up, in turn not flowing through the engine correctly thus contributing to increased wear rates. The excessive moisture levels combined with the soot and carbon in the oil produces sludge deposits especially in the valve cover.

The way to head off these problems is to ensure a good quality branded name SA 15W/40 of at least API CF4/SG specification is used, and the oil is changed every 5,000kms and filter at 10,000km or sooner. (Courtesy Australian Institure of Automotive Engineers.)

The oil that we use in our workshop is API SLCS specification SA 15W/50 which is 3 grades better than that listed above and is a semi-synthetic oil.

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