oil
provides cooling
feeds more easily from a central supply
washes away dirt
absorbs less torque
grease
better sealing against dirt
easier to contain and seal
longer periods without attention
allows simpler bearing desighs
application
lubricants are not generally interchangeable
they should not be mixed
Cleanliness is vital
No contaminants
Excess lubricant
Especially grease can cause too much heat and break down the lubricant
filtration
Always clean filters regularly