When pondering the difference between low mileage and high mileage used engines, the term night and day may come to mind. If all other factors are equal, obviously, a low mileage engine would be preferable. All miles that are logged on used engines are not the same. Mileage that has been accumulated by a little old lady that only drove to church on Sunday are certainly not the same as those put on by a wild-eyed, heavy-footed teenager on Saturday night. Engines that have been used for towing or those in rental cars are also considered hard miles. Other driving habits that put undue stress on used engines are over-revving an engine that has not been allowed to warm up properly, and operating an engine that is low on oil or having a crankcase that is overfilled.
Another important factor that needs to be considered when looking into used engines is maintenance. The type of motor oil used and its weight are extremely important. All manufacturers of internal combustion engines will have a recommended type and weight of oil and a timetable for changing it. Today synthetic oil is an available option and is highly recommended by automotive engineers and mechanics. On the positive side of the ledger, synthetic oil needs to be changed less frequently than regular oil and it reduces the amount of friction inside the motor. This reduces wear on internal components of used engines by allowing them to run cooler and also giving them better gas mileage and more power. On the negative side of the ledger is the price. Synthetic oil is fairly expensive in initial outlay, however, since it lasts longer than regular oil, the price is really a wash.
There are blends available today that are a mixture of synthetic and regular oil that will moderate your outlay of cash when investing in used engines. There is a school of thought that once synthetic oil is used, putting regular oil back in will cause the development of engine oil leaks. Consequently, when putting used engines back in service, it would seem to be a practical decision to use synthetic oil.
Another issue to be looked into is who has performed the maintenance work on the used engines in the past. One common belief is that the best source would be an automobile dealership. This could certainly be the case, however, some automobile dealerships refer to their service departments as the monster-in-the-rear for good reason. If the previous owner took care of their own maintenance on the used engines and had a fair amount of mechanical ability, this could be better. The thinking here is that an owner would use more tender loving care for their possessions than some technician in a repair shop. Of course they would need enough common sense to know their limitations. One of the best ways to determine their level of competence would be the existence or absence of a well used workshop repair manual, specific to make and model.