Is it appropriate to use the term "mileage" to refer to distance that is not measured in the literal units of miles? For example, would you say that a car "has a lot of mileage on it" in a country that measures distance in kilometers? Is there a more unit-neutral term that is better to use universally?
Mileage can certainly be used without having to be associated with literal miles.
Freedictionary defines it as an informal noun, meaning usefulness, or how much service something has provided, or may provide.
Cambridge dictionaries defines it as an advantage that can be obtained from a situation
A person may get good mileage out of a situation, meaning that they made good use of, or made good benefit from a situation. My very old clothes dryer is over 15 years old, and although it now needs replacing, I would say I got excellent mileage from it.
In Australia (where we use kilometres) the term mileage is also used to describe the fuel consumption of a vehicle - I get better mileage from my 2.0 L diesel sedan than my 4.2L V8 street racer. It would still be acceptable to refer to a vehicle having low mileage, although in print (car ads) it is more often described as low kms or low ks.
Though derived from mile the unit of distance, mileage as an informal term has a broader sense meaning distance-covered-for-the-fuel-used, a sort of return-on-investment.
Mile really is about 'thousands' (in the context of distance); not '1,760 yards' as in use today in formal writing.
In informal usage, mile does not even reference a fixed distance, implying instead, 'a very long way or a very great amount' (ODO).
Of course, your mileage may vary.