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Originally Posted by smitty477
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I don't know when this was written but the performance claims referenced in the text are out of date, by decades. Also the description of base oil being distilled from crude oil is also no longer applicable. No modern base oil is made this way, hydrocracking and isomerization is nearly universal, i.e. the base oil is synthesized by breaking apart the molecules and reassembling them in a configuration suitable for lubricating oil.
While there are API CD oils SAE 30/40 still marketed, mainly in the less developed world, these are lower performance engine oils with no actual testing supporting their claims (tests are long obsolete and not available). Oh, and the recommendation against synthetic may be a hang over from early seal problems as well as the typical wide viscosity cross grade of synthetics which may not be ideal in older diesels.