As dust gets in the mechanism, the oil becomes an abrasive paste, which causes wear. The longer the clock runs in this condition, the more repair it will need. Many American clocks have very strong mainsprings which will run the clock for many years after the oil has gone bad, causing severe wear to pivots and pivot holes. When the clock finally stops, it will take extra work to bring it back to proper condition so we can guarantee it.
Previous repair work by unskilled personnel causes more work for us and will increase the repair bill. Often these problems are not visible until the movement is disassembled and cleaned. Less than one out of ten clocks we receive for repair are in such good condition that all the clock needs is oiling!