To directly address the OP's original question, however, I would suggest being quite comfortable with how to use 91.213 to determine whether or not you can fly with a given piece of equipment inop.
This. It's a hot button item for the FSDOs right now. Too many airplanes that aren't airworthy causing discontinued rides, is what I heard triggered it. There's also a good Advisory Circular on it.
I also liked the ASA Oral guide, but it's not comprehensive... it makes you think about other things that could be asked. I like the format.
One of my DPEs was a systems trainer. He likes systems. At the Commercial level he'll ask about failures and a generic system overview of how it all works and what you can do about it from the cockpit. At the CFI level, you'd best be ready to teach all that detail.
Commercial privs is always a hot button during the regulatory questions. You'll also plan a longish XC and then be asked bigger picture questions than a Private candidate about why you chose that route, airspace is always fair game, and probably a scenario that touches on weather, diversion, and maybe some off-loading of one passenger and loading another at an intermediate stop. In other words, an abbreviated long XC discussion about stuff you might see that's different than a typical private flight.
Speaking of Advisory Circulars, that's a topic that often gets little love in Private checkrides, but Commercial examiners might ask things that pulling out an AC might be an excellent way to answer them. Browse the AC catalog and print off some common ones or have them available on the iPad, whatever.
Know WHERE to find stuff in the FAR and AIM. Spend some time in each. If a question is really "out there" but you know it's covered in the AIM, say so, and that you'll look it up. Sometimes all they're looking for is that you know the topic is there, other times they're having you look it up to set up a scenario or question.
There's SO much information available from FAA, if you're well versed in what's out there in it all, and can flip to the generally right area and find it, that's often part of the "stretch" type questions in the Commercial ride. Mastery of knowing where to find stuff shows you really studied and it often helps the oral go very smoothly. The examiner can tell in five minutes if you've really put in the time with the books. That becomes more important and detailed at the CFI ride, but it never hurts at the Commercial ride.
One of my questions was an electrical system fault that required knowing exactly how the busses were separated on one aircraft. It was real-world enough that you needed to not be cold reading the electrical systems part of the AFM, but also had a feel of "looking for the practical answer from the cockpit and appropriate judgement" besides being about knowing that particular system.
Props are always seemingly on the list, too. Again, the examiner can tell if you really read and understood the prop system in only a couple of questions.
In aircraft with any kind of "interesting" systems, expect a question or two there, too. In the twin, I got a few questions about proper operation of the Janitrol heater in cold weather.
How do you use it? Are there limitations on using it? How much fuel does it burn per hour? Did you factor that into your fuel reserves for this flight? How do you reset it, if it overheats?"
These were all quick Q&A that took about as long as it took you to read them here.
But also expect scenarios.
Annnnd of course, whatver changes the new Commercial ACS brings as FAA retires the PTS this summer, supposedly.
