How do you select between grease and oil lubrication for bearings?

Grease is preferred for sealed bearings, low-medium speeds (DN value < 300,000), infrequent relubrication, and simple housing design. Oil is required for high speeds, high temperatures (>120C), when heat dissipation is needed, or when other components need oil. Calculate DN value (bore in mm x RPM) to determine suitability. Grease fill should be 30-50% of free space; overfilling causes overheating. Consider operating temperature, contamination, and maintenance access.