Some laboratory studies have evaluated the oscillation mode of ultrasonic scalers. None of them recorded its influence on calculus removal and quantified dental hard tissue loss. This study aimed to compare the performance of a magnetostrictive ultrasonic instrument at different power settings in vitro in relation to the tip oscillation activity. The oscillation activity of the straight Slimline® insert in the Cavitron® ultrasonic scaling device was analyzed at five different power settings with the help of two laser vibrometers. The performance of this instrument was tested on 60 roots of human single-rooted teeth. Twelve roots each were randomly assigned to be instrumented at a given power setting. Every root was instrumented for 120 s at a standardized instrumentation force of 0.1 ± 0.05 N. In addition, another 30 periodontally involved roots with subgingival calculus were instrumented accordingly to assess the calculus removal potential. The surface characteristics after instrumentation were analyzed under scanning electron microscope. The instrumentation at minimum power setting resulted in an mean increase of the root surface roughness of 0.18 ± 0.28 compared to 0.51 ± 0.48 at maximum power setting (P = 0.0327). The loss of dental hard tissue amounted to 11.37 ± 3.64 at minimum compared to 23.37 ± 15.76 at maximum power (P = 0.0010). The higher the power setting, the more calculus was removed. The values of the latter ranged between 4.04 ± 1.87 and 11.26 ± 4.66 mm² of cleaned dentin surface area (P = 0.0065). At lower power settings, a more favorable relation between cleaning ability, loss of dentine, and surface roughness was found.