SN PLUS Motor Oils – Calcium Down, Magnesium Up

The SN PLUS Service Category was introduced as a supplement to API SN specification in 2018 to mitigate low speed pre-ignition (LSPI) events in Turbocharged Gasoline Direct Injection (TGDI) engines. Such LSPI events can cause engine knocking, decreased efficiency, and in the worst case, catastrophic failure due to cracked pistons.

As seen by the differences in the API SN and SN PLUS samples examined by PQIA over the past year, the SN PLUS formulations in the samples show lower calcium and higher magnesium levels than API SN. Whereas the average level of calcium in the API SN samples examined by PQIA is 2,060 ppm, the average of the SN PLUS samples examined to date is 1,217ppm. This represents a decrease of 41%. The average level of magnesium, however, has moved up to 691ppm, over 30 times above a negligible base of 22ppm.

Calcium and magnesium sulfonate are the most commonly used detergent additives in motor oils. Although calcium had been favored for decades, industry research has shown fewer incidents of LSPI occurred by re-balancing detergent chemistry to reduce the level of calcium and increase the level of magnesium sulfonate in the additive package. Calcium has been shown to promote LSPI, while magnesium has little impact good or bad. For more information on this topic, and other additive solutions to mitigate LSPI, see related links below.

The concentration of magnesium in the fifteen API SN PLUS samples examined by PQIA ranges from 373 to 1,445ppm, with an average of 691 ppm.

It is important to note that although there is a clear increase in the amount of magnesium in API SN PLUS samples tested and lower levels of calcium, and that calcium is an antagonist and magnesium helps mitigate LSPI, it would be inappropriate to draw inferences about the performance of an SN PLUS motor oil based solely on the level of magnesium in a sample tested.

We bring this to our readers attention since it is not unusual for PQIA to receive calls from consumers who focus in on one particular element or physical property in a sample with the belief that the higher the number the better. Although this could be true, within limits, the additive systems used in motor oils are a carefully balanced mix of functional chemistries. Whereas some formulators may choose to significantly favor the use of magnesium, others may look to strike a more modest balance between calcium and magnesium. Further, there are also other options available to address LSPI, including the use of titanium and molybdenum-based chemistries.

With that, the take away from the stats and facts about the SN PLUS samples examined by PQIA is that there is a significant change in the balance between calcium and magnesium in SN PLUS as compared to API SN. Based on the data, the typical level of calcium and magnesium in an SN PLUS will be in the range of approximately 720 to 1,430, and 375 to 1,450 ppm, respectively. Although there are other formulation options available to address LSPI, the samples PQIA has examined to date indicate the preferred approach is by way of rebalancing the calcium and magnesium levels in favor of more magnesium and less calcium.


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