There are a lot of comments made by people who have little regard for the American Petroleum Institute (API) and the specifications for motor oils with regards to motorcycle engine oils. In my opinion the API has a bad reputation despite the usefulness of the specifications is undeniable and there is no downside to using them as a performance benchmark.
API engine oil specification names, or “Service Categories”, start with an “S” followed by an alphabetically progressive designation to indicate the level of performance. API SA, SB, SC, SD, and so on until the most current category which, at the time of writing this, is API SN. The API systematically makes old specifications obsolete and ceases to issue licenses for them. The currently active and licensable categories are SJ, SL, SM, and SN.
You can read in detail the API’s own guidelines about each category and what they are suitable for. They are, of course, in reference to automobiles and there are exceptions to their guidelines, but the information is useful nonetheless.
There are two common complaints about the API specifications that I see often. The first one is that the environmental considerations for catalytic converters limit certain additives. Zinc dialkyl-dithiophosphate (ZDDP) in particular is referenced as being eliminated on the modern API specifications. The second common complaint is that the specifications are written for automobiles and do not take motorcycles into consideration. However, there are major flaws in both of these arguments.
The first complaint regarding ZDDP limits is a bit misdirected. The API actually limits phosphorus, which is only a part of ZDDP. For more in depth information of ZDDP and other anti-wear additives, you can read my article on them here: Anti-Wear & Friction Modifiers
The actual phosphorus limit varies for different viscosity grades, but most people don’t realize that the most common grades for motorcycles (SAE 40s and SAE 50s) are not restricted at all for phosphorus content. The only common grade used in motorcycles that does have limits is 10W-30. Since ZDDP is the most common source of phosphorus, and ZDDP becomes corrosive somewhere between 1500 ppm and 2000 ppm, this 1200 ppm limit is not a hindrance for formulators. For modern formulations, there are also many alternatives to ZDDP that do not contain phosphorus. So with all of the modern additives at our disposal, decreasing the ZDDP content compared to the formulations of the past is not as doom-and-gloom as many people make it out to be .
The second concern regarding the automobile applications for which the specifications are written is a somewhat valid argument, but as I said above, the argument has it flaws. There is no doubt the specifications are not taking motorcycles into account for the performance requirements. However, this does not mean that the properties being required for automobiles do not relate to motorcycles. Deposit reduction, oxidization inhibition, anti-foaming, volatility, sludge reduction; these are just some of the performance properties guaranteed by API rated oils and they are universal to internal combustion engines. The conditions of an automotive engine may not be identical to a motorbike’s, but that doesn’t mean the benefits can’t overlap. The API specifications still give general performance minimums, and as the specifications progress, the performance continually improves.
The most commonly used engine oil service category for motorcycles is probably API SG. This designation has long been obsolete, but there is a perception that API SG oils have much better anti-wear protection because of the ZDDP content described above. Of course, newer categories need to pass all of the anti-wear performance tests that API SG oils did so there is no loss of anti-wear performance, but the myth still persists. Since it is so commonly referred to, I’ll use API SG as a benchmark in Figure 1 below.
Each category is either more refined using the same tests or passes a more severe test for each advancement. Deposit control increased four separate times. Oxidation reduction, anti-wear performance, corrosion protection, and foam control all increased three separate times. Sludge reduction and oil consumption improved twice and foam control was improved once. While those improvements happened over the years, phosphorus content was further restricted twice. As you can see, there is no point in which a later specification has a decrease of performance in any area.
There is rarely any groundbreaking performance increases from subsequent service categories, but with each advancement, the oils have progressively improved. The chart illustrates the point that regardless of any particular additive restrictions, performance has never regressed during specification advances. Wear, sludge, oxidation, deposits, foam, and corrosion are all improved in subsequent service categories even with phosphorus limits being reduced twice during that same time period.
So now, when you are comparing oils and weighing your options, know that if two oils have different API designations, there is no compromise in choosing the oil with the higher API performance category. The API designations are not some mysterious arbitrary rules that have no relation to motorcycles as some like to believe. They require very specific and extensive testing to verify their performance in many different areas of stress relevant to any internal combustion engine.
If you’d like to see them for yourself, Infineum (an additive manufacturer) has a fairly readable guide for the currently active API service categories here: