Can I use a diesel motor oil in a car?
The newest CJ-4 formulations are now limited in the amounts of Zn and P allowed, compared to the CI-4 formations most users are familiar with. If the oil says CJ-4 or LE or "Low Emissions", even if it also lists CI-4 or CI-4+, more than likely it conforms to the new, lower levels, and should be avoided unless you have a 2007 and later diesel which requires these oils. The verdict is still out in my opinion on the performance of a CJ-4 oil in our application. If you choose to use a diesel or mixed fleet oil, stick to an a CI-4 rated oil. Nowhere should the oil say for emission system protection or for use in engines equipped with particulate emissions filters. Swepco 306 15w40 is one such example of an extraordinary diesel oil we use in our performance engines. Swepco 306 has a healthy dose of boron and moly also for additional ant-wear response.

Worth noting, CI-4 diesel motor oils tend to have more detergents, and it has been determined in the SAE paper "Oil Development for Nascar" that overly detergent motor oils can block or "clean" the anti-wear films off of engine parts, that is one reason that these oils usually have high levels of anti-wear additives. SAE Technical Paper Series 2007-01-3999, Modern Heavy Duty Engine Oils with Lower TBN Showing Excellent Performance, also show that low detergent packages increase the effectiveness of film formation, just as in racing oils tend to have less aggressive detergent packages, which is another reason many be to consider a racing oil versus a diesel oil for your engine. The new ACEA E9 specification will be for a low Zn and P (ash) oil with low TBN, building upon this relationship to ensure that wear protection is not sacrificed for improved detergency or longer drain intervals.