OPS University: Oil Sampling

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A common question I receive is, “My sample shows my viscosity at 14.1 what does that mean?”

First, we need to understand that viscosity is the lubricant’s internal resistance to flow tested at a given temperature (usually 100 degrees centigrade for motor oil) in relation to time. Most people believe it is the thickness of the oil, while the oil may be thicker or thinner the viscosity still needs to be verified to test the internal resistance. Viscosity is the single most important physical property of any lubricant. The flow rate is determined using a kinetic method and results are reported a centistokes scale that is converted to SAE Grade for simplicity. Increases and decreases in the viscosity would indicate improper servicing, contamination, fuel dilution, lubricant additive breakdown etc. Heavy duty diesel engines usually require SAE grade 30, 40 or 50 wgt oil. The centistokes readings should be in the following ranges 30 wgt: 8.3 to 12.29, 40 wgt: 12.3 to 16.29, 50 wgt: 16.3 to 21.9; the higher the grade the higher the internal resistance and the slower the flow. Most oil sampling companies will advise oil changes if viscosity goes up or down a grade. Fuel and water will dilute oil lowering viscosity and soot, sludge, glycol, oxidation and heat will increase viscosity. If you install 5w40 oil, you want to keep the centistokes readings between 12.3 and 16.29 if you mix oil by adding a 10W30 or 15W40 the viscosity will obviously change therefore it is best to use the same grade of oil and avoid mixing.

Another question I hear all the time is about private branded oil and whether it is okay to use in diesel engines because it is much cheaper than the major oil companies products. The answer is always the same. First, you need to know what oil formulations are required by your engine manufacturer which will usually be stated in the owner’s manual as an API Service Category and SAE Viscosity Grade. Beware! The jug may say for DIESEL ENGINES but it may be for automotive diesels not Heavy Duty Diesel Engines.

The new question becomes, what is the API Category and how is it determined? Engine manufacturers and the American Petroleum Institute work together to define strict criteria and standards that oil must meet to protect the engine properly to ensure oils are properly formulated for various engines and duty functions. The American Petroleum Institute sets standards and certifies motor oils that are voluntarily submitted by oil manufacturers for marketplace sampling and testing. Oils that meet or exceed the standards are certified and may use the API Service Symbol on the oil containers. Therefore if the oil sold at a department store has the API Service Symbol on the bottle that conforms to requirements of your engine it is safe to use. In addition it should also be noted that the private branded motor oil was not produced by the department store but by a major oil company.

The next question is what does the CJ-4 SAE 15W40 on the bottle mean? The CJ-4 oil certification was introduced in 2006 for high speed, four stroke diesel engines designed to meet the 2007 on highway exhaust emissions and can be identified by the API Service Symbol on the oil container.

“C” = Commercial – Heavy Duty Trucks with Diesel Engines

“J” = The series identifier introduced in 2006. “I” was introduced in 2002, “H” 1998 etc.

"4” = Four Stroke Engines

SAE = Society of Automotive Engineers (Sets standards for viscosity).

15W – 40 = rating of oils ability to flow at certain temperatures. . For example if lowest outdoor temperature is 0 degrees Fahrenheit you need a 5W or 10W oil, less than 0 degrees 0W or 5W will allow the oil to flow as required.

Any information placed in the bottom section of the API Service Symbol identifies characteristics of the oil formulation. CI-4 Plus for example is formulated to provide higher level of protection against soot related viscosity increases and shear in diesel engines and may be used in place of CJ-4 oil. You would never want to use any oils with an “S” as the category as these are strictly for gasoline engines and are not formulated for the higher heat and soot generation of diesel engines.

If you have any questions about oil sampling, oils or bypass systems please send to me at:


  • oil filter
  • oil sampling
  • oil viscosity
  • oil recycling
  • conserve engine oil
  • oil analysis