I don't think it has anything to do with lubrication, although lubrication may be more suspect at the lower rpm's where lugging occur's.
Lugging occurs because the rpm of the motor and the amount of energy each piston is able to contribute in a cycle varies, as the motor is turning. Consequently at lower speeds, with more load, the piston and crank push down harder right after the fuel is ignited, then as the piston loses energy, the load is pushing back and it gets into a mechanical loading, oscillation cycle, which places all sorts of loads on the engine parts.
Since this is happening in a car with 4 cylinders, or more, it is really happening in 4 places in the engine, which goes into a hinky jinky oscilation, including the transmission, halfshafts, wheels and so forth. This oscillation puts stress and load on all of the drivetrain, which are jerky and in excess of normal.
It is sort of a self induced equivalent of revving the engine really high, then popping the clutch out, only it occurs multiple times per second, when the whole drivetrain gets into it's dance. The only way to stop it is to either back off on the throttle, or disengage.
On my Honda as long as I only use a light throttle I can run it at a lower rpm, but if I open the throttle up, it overloads the system and it does the hokey-pokey. Hokey-pokey is hard on all of the drivetrain and if you don't like replacing engine and drive train parts, it isn't a good idea
Some engines are better able to cope with the effect. Heavier flywheels, thicker castings, special oils, just general 'beef' mitigates the pounding into a banging that can be tolerated for hundreds of thousands of miles.
Not that I operate that way exclusively, but probably more frequently than most might consider prudent.
the gas and o2 burn at a certain speed, if the piston is allowed to move too slowly, the pressure in the cylinder is greater. I have a Honda, they are notorious for not having any bottom end. So I don't like going below 1500 in 4th, and about 1750 in 5th.
When I drive through my neighborhood with a manual transmission, I use 5th gear. The power the engine at idle is enough to maintain constant speed at ~25 MPH. Scanguage will show around 80 mpg! Note to keep idiots from hurting them selves: obviously you can't use more than ~5% throttle, or it will lug.
Scanguage even shows decreased FE when lugging. Gear selection depends on the throttle position.
Cars aren't lugging at an idle at 700 RPM with the throttle closed.
85 Chevrolet. 30 MPG or bust!