PM is maintenance on a calendar or a meter, not on evidence of wear: change the oil every 3 months or every 500 hours, whichever comes first, because that's what the manufacturer specifies for that duty cycle. It trades a small amount of unnecessary work (replacing a part that had life left) for a large reduction in unplanned failures.
It's the foundation most maintenance programmes are built on, and the source manual for the intervals should always be the OEM's own service documentation — generic industry intervals are a reasonable starting point, not a substitute for what the manufacturer actually specifies for your exact model and duty class.
PM vs PdM vs CBM
Preventive maintenance runs on a fixed schedule regardless of condition. Predictive maintenance and condition-based maintenance both use real asset data instead — the practical difference between them is timing horizon: CBM reacts to a threshold being crossed now, PdM forecasts when it will be crossed.