DIAGNOSIS The adjustable bar oil pump is set too high, or oiler plunger (if you have that model) is stuck partially engaged.
FIX On the MS 170, there's a small screw on the bottom of the saw, near the clutch cover, that controls the oil pump output. Turning it clockwise (toward the minus sign) reduces oil flow. Start with a quarter-turn reduction, run the saw, and check. You want a light line of oil flung off the chain, not a spray.
IF THAT DIDN'T WORK The oil pump itself might be worn or damaged, allowing too much oil past its metering groove. That requires pump replacement.
HEADS UP Using thin, non-Stihl bar oil in cold weather can also cause excessive flow because it's less viscous. Always use the proper oil for the temperature.