Quote:
Originally Posted by JonL
API would demand that all oils that meet the specification be fully interchangeable and have no adverse interactions if mixed.
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There is no such requirement. API only tests to define, certify, and monitor engine oil performance deemed necessary for satisfactory equipment life and performance by vehicle and engine manufacturers.
http://www.api.org/certifications/engineoil/pubs/upload/1509_16thedition042007.pdf