Chris A,
Good question. I have a 95 PSD with a Cat. Conv. and it definetly takes 15W40 CI/SL oil. So to me if an oil has both ratings I don't see a problem with using it in an auto engine.
I personally have never seen this tested and in writing. I do read alot of people say not to do it because of the ash content, but no documented facts.
An API rated CI/SL 15W40 should not be a problem. I have seen some 350 cu. in. GM engines that the lifters would go nuts on a cold morning using this weight oil. That is the only drawback I personally see. Also the Phor. content of the HD oils are being lowered which is another plus. JMO