There is no proof that idling causes lifter failures, just speculation based on the amount of cop cars with lifter failures so people start jumping to conclusions and think that cop cars = iding = failure. There is also no proof that idling doesn't cause failures, but there are other theories; cars that idle extensively are required to use the "extreme duty" maintenance cycle which is changing the oil at 320 hours. If a cop car runs 24/7, that means it should be getting oil changed every 2 weeks. If it runs half a day it needs to change every month. Many police depts don't follow that schedule, so the bad/dirty oil starts to form deposits on the needle bearings of the lifters and they sieze up, eventually the lifter takes out the cam lobe and its game over.
There are Rams that have over 1000 idle hours with no lifter failure (see "high mileage edition" in the "truck central" youtube channel), and some that fail with < 20,000 miles.
The lifters are primarily lubricated via pressurized oil galleys, not crank splash. This design has not changed since 2003 when the hemi was re-introduced, so if we needed to depend on crank splash for lifter lube then the pre 2009 hemis would be having lifter failures as well, which they don't; therefore it's not a crank/splash problem.
We have also seen an analysis of failed lifters/cams from a metallurgy specialist and he basically said that oil was present, but was not effective ("insufficient lubriscosity" IIRC).
Basically the best prevention is: change your oil frequently (< 8000 miles), use 100% synthetic, and if you're really into it (but ok with voiding warranty) run something like Redline 5w-30.
Another theory; the hemi's were originally designed for 5w-30, but dropped to 5w-20 to improve fuel efficiency. Nothing in the design changed when they dropped the oil weight, and the manual for a period of time even still "allowed" you to run the 30 weight. This theory says that the 20 is too thin and that would be partially backed up by the specialist report mentioned above.