I've spent 35yrs in engine design/development. A couple of things I'd offer.
Forget the stuff about "not lugging your engine" (i.e. running at low speeds+high loads). This dates from early days when oil systems, bearing materials and oils themselves were poor. You can run a modern engine at full load and 1000rpm with no lubrication issues, it won't pull much but it won't harm it.
Having said that, generally you don't gain anything by dropping much below typically 1500rpm at any significant load because the thermal efficiency drops off quite rapidly at low speeds (heat loss increases as the cycle times get longer). For many petrol engines something like 1750-2000rpm would be a nice range to aim for if economy is your priority.
Running at low speeds/light loads tends to cause more issues than running at mid speeds/loads. Repeated cold starts and short runs when it doesn't really heat up are the worst. If you tend to use the car for short trips, it really does help things to give it an occasional decent run, get it hot, and use plenty of rpm and load. One feature is that valves rotate gradually, this helps even out deposits and wear, ensuring good gas sealing. On many engines this tends to start happening at around 2500-3000rpm (varies with design of springs etc), so taking it up to perhaps 4k rpm for a short time every now and then and holding it for at least 30sec (maybe half a mile) really is useful in getting the rotation going. Also surface temperatures rise directly with engine speed, spark plugs and exhaust valves etc, and this helps clean them of deposits. Motorway use is pretty engine friendly.
The "Italian tune-up" may be a bar-stool story, but it really does have truth in it.
