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I had a 6.7L Cummins diesel in a Dodge pickup truck and it came with an air/oil separator. The owner's manual called for it's replacement at 67k miles. Interestingly, the ECU monitored crankcase pressure and would display a message if the AOS failed.
Cummins used an AOS because it provided better oil control (less oil burned). On recent diesel pickups the entire exhaust gas stream is run through a particulate filter, which is periodically burned off at high temperatures in a process called regeneration. The filter is quite expensive and must be warrantied for EPA emission system compliance. The main culprit in clogging up the filter is not carbon soot because that can be burned off. The problem comes from non-combustible ash and a big element (no pun) is zinc anti wear ingredients in the motor oil. The AOS contributes to the particulate filter life by reducing the amount of motor oil consumed. The motor oil itself was also reformulated to use less zinc (ashless oil).
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