Overkill is right about the low or minimal throttle plate opening. One of the larger losses in gasoline engines is due to engine vacuum at part throttle. Both cylinder deactivation and additional transmission gear ratios address this. With 6 to 10 speed automatic transmissions cylinder deactivation doesn't add much value anymore.
Also, last month I was looking at purchasing a Ram 3500 and noticed that they still are having some issues with cam/lifter failures. Like the GM LS engines the path for splash lubrication from the rod journal is poor, causing problems mainly in police cars that idle most of the day. The other issue with Ram is that the deactivation lifter uses a soft internal lifter spring (intended only for low rpm) for cylinder deactivation that re-activates by having oil pressure activated pins lock this feature out. Rev the engine up quickly, at the pins likely take too long to activate causing the lifter to float, i.e. loose contact with the cam, followed by harsh crashing contact.
It appears that even deactivating the system via a tuner, along with limiting the idle time reduces or possibly eliminates Hemi cam problems. I think the GM LS engines are similar, but my personal GM experiences have been horrible so I'm not sure about that engine.
The HEMI lifters aren't oiled by rod journal splash, there's a big chunk of the block in the way, they are oiled by the pressurized oil provided to the lifter bores which runs down the lifter body and onto the roller/lobe.
It's a materials quality issue with the lifters, not a problem with the engine. It also affects non-MDS lifters and if you do have lifter failure, it's a crap shoot whether it's an MDS one or a non-MDS one that the roller hardening was breached on, and seized up. This is why they've revised the lifters like 7 times. This issue also happens on non-MDS (Hellcat and 6spd cars) engines, though it has been greatly reduced overall apparently, since one of the more recent lifter revisions (both of my vehicles would have the updated lifters).
Basically, if you are one of the (relatively rare) unlucky ones to have improperly hardened rollers/pins, it's only a matter of time before that surface is breached and the needles start to catch, eventually creating enough of a groove that they pile up, the roller stops rolling and away you go. Because cop cars accrue an insane amount of time (hours) on the engines relative to actual miles, it seems to happen quicker on them, but in reality, it just takes more miles for a civilian to accrue the same amount of hours.
I don't believe the # of gears really has any impact, the HEMI has been backed by an 8spd for ages now. Load is load, if you are cruising down the road and using 50HP to maintain your speed, if you turn that engine into a 4-banger, it's going to require more throttle angle to create that 50HP, regardless of whether that's a 4spd or an 8spd, which in turn reduces pumping losses and pushes the engine further up into the efficiency curve, improving gas mileage.