City miles on a vehicle are considered worse than highway miles because when you're just doing city driving the engine is not getting up to temp to keep condensation out of the oil.
Also when you drive in city traffic and have to constantly stop and then get the vehicle started going again the transmission has to then work to shift back into gear which creates more wear and tear on the shifting gears inside the transmission.
When you're driving at a constant highway speed the transmission stays in that gear for a longer time and also the engine stays at a hot enough operating temperature to evaporate the condensation that can get into the oil naturally.
Highway miles are much better on a car than city driving miles with stop and go driving.
At highway speeds the engine gets up to temperature better which helps burn off the moisture from the combustion process.
That keeps the oil in better shape and helps keep the engine lubricated better and at highway speeds the engine runs cooler due to air flow over the radiator and engine.
When driving around town the air doesn't get to the radiator or engine properly so the engine can run hotter as a result.
The hotter the oil the better the oil is at lubricating and prevents oil sludge buildup as well.
Highway driving is also good for an engine because the cylinders get hot enough to burn carbon deposits off and assuming you don't change speed too often the engine stays in lean burn mode so they don't form in the first place.
The transmission also has to shift less and so the less often the transmission has to shift into and out of speeds and gears the longer the transmission will last.