API CLASS RATINGS APPLICABLE TO MOTORCYCLES:

SA through SE OBSOLETE -- Use SF/SG or SH unless you find motorcycle oil which still meets the original rating that your bike calls for in the owner's manual. I am not familiar with any SA/SB/SC/SD/SE rated oils still on the market, but there may be some available for special vintage class bikes (bikes built between 1910 and 1970).
    
SF & SG   TECHNICALLY OBSOLETE FOR CARS, BUT STILL IN COMMON USE FOR MOTORCYCLE OILS -- You can find motorcycle oil which still meets the SF/SG rating (such as offerings from Castrol, Mobil, Royal Purple, Amsoil, Bellray, Motorex, Motul, Repsoil, etc). If your bike calls for that in the owner's manual, there are still a large number of motorcycle-specific motor oils on the market that are SF/SG rated, including every motorcycle oil which meets the full JASO-MA specification. Most current motorcycle engines still call for SF/SG rated motor oils. If you can not find an SF/SG rated oil and that is the rating called for, use an SH or SH+SJ rated oil -- but do not use API SJ, SL or SM!
    
SH   Technically obsolete.  This oil specification is used by some motorcycle manufacturers, and you can still find many SH rated motorcycle oils on the market. Do not use if your motorcycle calls for API SJ/SL/SM rated oils (only a few do).
    
SJ   Technically obsolete.  For 2001 and older automobile engines. An "Energy Conserving" motor oil (something good for late model cars, not most motorcycles). Note that this standard reduces the phosphorus contents of the motor oil, which is not necessarily a desirable reduction for most motorcycle engines. Check your motorcycle owner's manual specifically for recommendation of an SJ-rated motor oil before using it in your motorcycle, or stick to using SF/SG-rated, SH-rated or dual-rated (SH+SJ) motorcycle oils instead. Do not use unless an API SJ rating is specifically called for by your motorcycle manufacturer.
    
SL   Technically obsolete.  For 6/2001 - 11/2004 automobile engines. An "Energy Conserving" motor oil (something good for late model cars, not most motorcycles). Note that this standard reduces the ZDDP (zinc and phosphorus) additive contents of the motor oil even further than SJ rated motor oils, which is not a desirable reduction for most motorcycle engines. Check your motorcycle owner's manual specifically for recommendation of an SJ or SL rated motor oil before using an API SL rated oil in your motorcycle! Do not use unless an API SJ or API SL rating is specifically called for by your motorcycle manufacturer.
    
SM   NEW/Current.  For all 12/2004 and later automobile engines.
    
       An "Energy Conserving" motor oil (something good for late model cars, not most motorcycles). The API increased the standards for anti-foaming, increased detergent levels, and improved low-temp performance, while lowering the ZDDP package contents again. API SM now replaces API SJ and API SL -- but SM-rated oil is still not suitable for most motorcycle engines (just as SJ and SL rated ones weren't), especially virtually every air-cooled, oil-cooled and air-oil cooled motorcycle engines and ANY motorcycle engine designed prior to 2001 (such as Suzuki Katana 600/750/1100, Suzuki Bandit 600/1100/1200). Check your motorcycle owner's manual specifically for recommendation of an SJ/SL/SM rated motor oil before using API SM rated oils in your motorcycle! Do not use unless an API SJ, SL or SM rating is specifically called for by your motorcycle manufacturer.

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Is SL or SM better than SF/SG for my Motorcycle?
   API's Standards Rating is influenced by government mandates (such as pollution control), and thus oils meeting newer ratings do not necessarily perform better (or even the same as) oils meeting an older rating, depending on your motorcycle engine type...
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Comments from the API in repsonse to our query, 11 Aug '05:
(Note this correspondence has been trimmed for length)

   Higher performance engine oils such as API SJ thru SM will contain some level of friction modifier. While the friction modifier improves fuel economy, it is not compatible with wet clutches used on motorcycles. The friction modifier causes the wet clutch to slip. This is especially true if the motorcycle manufacturer recommends using only engine oils carrying API SF or SG Service Categories.
   API recommends following the Original Equipment Manufacturer's (OEMs) oil recommendations. Typically an API SJ through SM engine oil would include some friction modifier. Only if the [motorcycle's] OEM indicates that higher performance engine oils such as API SJ, API SL and API SM it is possible to use these engine oils in these motorcycles.

Regards,
Dennis L. Bachelder
American Petroleum Institute
1220 L Street NW
Washington DC 20005 USA

API or JASO?
   If your motorcycle manufacturer specifies both an API rating (such as SH), and the JASO MA rating, use only motor oils that satisfy both of the standards listed (the packaging for the oil will carry symbols for both if it meets both standards). Do not use an oil that only meets one standard but not the other, if both standards are specified by your owner's manual or the motorcycle manufacturer. Note that Shell Rotella oils are not JASO MA certified as of the time I write this due to the high sulfated ash content (JASO spec is maximum of 1.2% by weight). If your motorcycle engine was build between 1980 and 1998, it will not have a JASO specification, but will benefit from using a JASO MA spec oil (in conjunction with whatever other specifications the manufacturer requires).