Jacobs were the first makers of engine brakes, thus "jake brake" is the generic name for a variety of brands. Engine brake versus exhaust brakes. Smaller engines use exhaust brakes...actually and engine "retarder". Main purpose is safety, and keeping your service brakes cool. Preserves service brakes, which in a large coach can be a $2-$3K job.
Signs going into towns that say "Don't use Engine Brakes" or the like are never applicable to exhaust brakes.
Blow-by: An agreement between Cummins and PacBrake resulted in an increase in orifice. Reason: Oil crossover problem -- Oil spots on back of coach and on the toad. The back pressure slips past the turbo seal, gets into the base of the engine, increasing pressure in the engine resulting in blow-by. Solution has been to lessen the backpressure by increasing orifice size, and that results in decreased performance from the PacBrake.
Engine RPM versus performance: The higher the rpm, the more efficient the braking power. Brake horsepower increases with increased rpm. Exhaust brake is a "stove pipe damper". Flapper closes prevents the escape of engine pressure. Like putting your finger over the end of a bicycle pump. It attaches to turbocharger, and when activated restricts exhaust from engine. More RPM means more backflow through the engine means more retarding power.
There is nothing you can do to harm the PacBrake. Engage or disengage it any time, as often as you like, and it's worry free. You can leave it on all the time. Only reason to turn it off is to increase "coasting", and thus fuel economy. Recommends leaving the PacBrake switch on all the time. Only time you'd consider turning it off is on icy roads.
Exhaust brake maintenance. It's physically located just below the bed in the coach. Needs lubrication periodically. If it says "Contains petroleum distillates" -- don't use it. If you're in the salt air, stored, and firing it up periodically, you're not exercising the PacBrake, because it does not engage until you're in second gear in most applications. [Permatex Superlube] Dry or wet silicone okay. Graphite base okay too. Squirt it down the shaft -- each end of cylinder and into the shaft. "Just drown the thing".
Cruise control with PacBrake? Should have no impact on fuel economy.
Warranty on PacBrake is 5 yrs/100,000 miles.
Cummins is emphatic that the engine will "live in the red zone" in a decel mode (i.e. when it's downhill). So 2800 rpm in decel will give optimal braking power w/out harm to the Cummins engine.