How Do I Read the Engine Oil Bottle Numbers and Letters?

Check Engine Oil
It is time to change your oil, or maybe you are just curious. What exactly are the numbers on all the oil selections at your auto parts store? You should know what kind of oil is appropriate for your car and your owners manual should have that information in the front of the manual.  Some new engines require synthetic oil, while older engines use conventional oil.


All oils are intended for an application and in general are not interchangeable. So what are we looking for in an oil? What does it do?  The oil must be thick enough to adhere to the components as it passes by to provide adequate lubrication. At the same time, it must be thin enough to easily flow through the system.


Most oils on the shelves today are “Multigrades”, which simply means that the oil falls into 2 viscosity grades (i.e. 10w-40 etc)
In a 10w-40 for example the 10w bit (W = winter, not weight or watt or anything else for that matter) simply means that the oil must have a certain maximum viscosity/flow at low temperature. The lower the “W” number the better the oil’s cold temperature/cold start performance.

The 40 in a 10w-40 simply means that the oil must fall within certain viscosity limits at 100°C. Once again the lower the number, the thinner the oil: a 30 oil is thinner than a 40 oil at 100°C, etc.

Service Classifications: A two-letter code is listed on the motor oil label that tells you which types of vehicles the oil is designed for. It will begin with either an S for gasoline engines or a C for diesel engines. The second letter tells you which model years the oil is designed to work with. For the letter A, the oil meets requirements of vehicles that were developed before 1930. The classification of current vehicles is N, so that the two-letter code should read SN for owners of relatively new, gasoline powered cars. This is mainly important for classic cars.

Gasoline – All specifications prior to SL are now obsolete and, although suitable for some older vehicles, are more than 10 years old, and do not provide the same level of performance or protection as the more up to date SL and SM specifications.

Diesel – All specifications prior to CH4 are now obsolete and, although suitable for some older vehicles, are more than 10 years old and do not provide the same level of performance or protection as the more up to date CH4 & CI4 specifications.
If you want a better more up to date oil specification then look for SL, SM, CH4, CI4

A lot of the other information are meaningless and just marketing blurbs. Be wary of statements like “synthetic blend” if you are looking for a fully synthetic oil as this will merely be a semi-synthetic.

Like everything in life, you get what you pay for and the cheaper the oil the cheaper the ingredients and lower the performance levels.

Symptoms of Using the Wrong Oil

If you use the wrong viscosity oil, you won’t notice much difference as long as it is close in grade. However, the engine will wear down sooner and may provide lower performance. Mixing synthetic motor oil with conventional oil and vice-versa can damage internal engine seals and gaskets and lead to overheating problems.

Having a professional service your vehicle can avoid issues of using the wrong kinds of oil. Be sure to pay attention to your mileage and when the last time you changed your oil was.

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