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API motor oil classification "SN" and "SN PLUS"

6722 Views 6 Replies 4 Participants Last post by  adc
Not long ago, I was in an online discussion in which, during the discussion, the merits of API SN classification oils as compared to the more recent API classification SN PLUS (SN+) came up. The concerns discussed by these posters centered around intake valve deposits and pre-ignition, or more specifically a type of pre-ignition known as High Load / Low speed pre-ignition (LSPI), often shorten to just Low Speed pre-ignition , or called stochastic pre-ignition (SPI) .

One of the participants opined that the principle benefit of using API SN PLUS motor oil was the reduction of deposits on intake valves and, in particular, those of Gasoline Direct Injection engines. This did not quite make sense to me, based on what I knew were the causes of intake deposit problems in years past, but I decided to look into it for my own edification along with updating myself about LSPI.

When my recently purchased 2010 model year Forester was built in August of 2010, Subaru listed in both the Owner's Manual and the applicable Service Manual that the American Petroleum Institute (API) Service Category “M” (SM) grade motor oil stating the phrase, “Resource Conserving”, with an Society of Automotive Engineers (SAE) viscosity range of 5W-30, and a International Lubricant Specification Advisory Committee (ILSAC) classification of GF-4 motor was the primary engine oil requirement.

In October 2010, the API service category SN was introduced and SM became obsolescent and is currently considered suitable only for gasoline vehicles model year 2010 and older. API SL, SH, SJ were previously rendered obsolescent in turn and are only suitable for vehicles built to a certain date as well. All API oils SH and older are currently obsolete. At the same time ILSAC GF-5 replaced GF-4 as well as all other previous GF classifications.

So, when Subaru built my Forester they specified SM “Energy Conserving” , SAE 5W-30, ILSAC classification GF-4 as the preferred motor oil for my Forester. The only alternative recommend if the ambient temperatures never drop below -4 Fahreheit (-20 Celsius) and 5W-30 is unavailable, is 10W-30 and 10W-40 as long as the oil has the same API and ILSAC specifications.

Following Subaru's reliance on API and ILSAC classifications, the current motor oil specifications for my Forester, in what ever brand of conventional, semi-synthetic, or synthetic motor I prefer, would be as follows:

SAE: 5W–30
“Resource Conserving”

Subaru does not reference, in the literature I have seen to date, European motor oil specifications often noted on oil bottles by the French language acronym, ACEA. The ACEA (Association des Constructeurs Européens d'Automobiles) or in English, the European Automobile Manufacturers' Association, is an organization that represents the 15 most important European motor vehicle manufacturers. Since Subaru does not reference ACEA oil standards, to the best of my current knowledge, I will not make further reference to the ACEA.

Here are two links for the ACEA if you would like more information:

Many manufactures have their own oil quality standards, Ford, GM, Honda, and BMW are just a few examples. I have been exclusively a VW / Audi owner for a very long time. VW has about 11 different “VW Oil Quality Standards” that are routinely updated through Technical Service Bulletins (TSB). The last time I looked at a Oil Quality Standards TSB there were one hundred or more brands of motor oil sold worldwide that met VW Oil Quality Standards listed. Such standards for any particular VW Group specification may, or may not, also meet API and ILSAC classification standards.


“A specification is an interface between an oil's physical and chemical properties and the car manufacturer's requirements. [Not all]car manufacturers nor [sic] lubricant producers are listing all the relevant properties in their data sheets. Car manufacturers combine a set of expected properties and test results in a specification while oil companies have their products tested for those requirements and show the earned specification or approval on the product as proof that their product is fit to be used where that particular spec is demanded. So specifications are codes, which we use to match oils with cars. By choosing an oil based on a specification we make sure that we choose the right oil for our car.”

In my view, Subaru has Oil Quality Standards, but they are not ones that are specifically developed by their own engineers, as is the case of the Volkswagen Group of companies, for example. But rather those develop by the API and ILSAC.

As for the case of SN and SN Plus API classification oils, according to the API, the SN PLUS classification was specifically designed to address the phenomenon of High Load / Low Speed pre-ignition (LSPI) while still providing the regular benefits of the SN classification. There are three versions of SN classification motor oil. Please see the attached files.


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I would not put a resource conserving oil in any of my cars. SM, SN, doesn't matter, it's not going in.
I would not put a resource conserving oil in any of my cars. SM, SN, doesn't matter, it's not going in.
Why is that?
Here's in an excellent video on the problem with intake valve carbon buildup in Direct Injection engines. If you have a 2019 Forester and are not familiar with this issue you may want to give this video a watch:

Another reason I'm glad I have a 2018.
Why is that?
Background: I own vintage Subarus and I race my 2002 WRX & '87 RX. YMMV, and it probably will...

Resource conserving oil is called that because thinning the oil is a simple way to reduce internal engine friction and boost fuel efficiency. These benefits come at a price, which is the oil in much less robust and is intolerant of high temperature/high shear.

Performance Subaru engines were never really designed for high performance. They're essentially band-aid solutions to boost output, but the design is very similar to their more vanilla models. When compared to engines with similar architecture (Porsche) there is not enough bearing surface area for the specific output these engines are capable of. The cases also like to shift around under power because they've only got two dowels to center them, allowing the main bearing bore to get out of round.

All this friction and inadequate surface area leads to high oil temperatures that quickly kill your average oils, shearing your XX-30W oil down to a 20W, when the engine really would like a XX-40W oil at those temps.

Besides the physical degradation of the RC oils in extreme conditions, they've been stripped of their anti-wear additives, like ZDDP. The older Subarus need these additives due to their flat tappet valvetrains.

Luckily the more boutique oil manufacturers still produce oils suitable for my applications.
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When discussing engine and vehicle manufactures' motor oil requirements there is always the risk of exposing oneself to the B.O.C. (Boutique Oil Cult) and the resulting conflating, convoluting, and utterances of received wisdom and pit lane gossip that will follow. And when this then happens ...... I often remember sailing on a Replenishment Oiler in the Summer of '76 listening to Blue Oyster Cult's - (Don't Fear) The Reaper and, well, all is well once again.

But back to the matter at hand.

If the time is taken to read the supplied API SN PLUS documentation, the only real "stand outs" to separate the three API SN oils, are that SN "Resource Conserving" oils are required to retain minimum levels of phosphorous and the other two are not, and to demonstrate fuel consumption saving rates on a test engine that the other two SN oils are not required too either. Please note the foot notes on the shear test.

The following ASTM tests are tests that SN "Resource Conserving" oils are required to meet and pass that the other two SN oils are not required too.

ASTM D7589 (Sequence VID) (Please see the attached.)

ASTM D7320 (Sequence IIIGB) -or- ASTM D8111 (Sequence IIIHB)

ASTM D7563


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I really lost the jist of your question. But let me say this: The EJ engine can run of a mixture of cat piss and any 5W-30 oil. You are overthinking this. Life is to short.

btw I have run many used oil analyses for 3 EJ engines and seen hundreds more.
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