Yamaha YBR125ED Owner's Manual page 56

5. Remove the air filter elements from the guide and clean
them with solvent. After cleaning, remove the remaining
solvent by squeezing the air filter.
6. Apply recommended oil to the entire surface of the filter
and squeeze out the excess oil. The air filter should be
wet but not dripping.
Recommended oil:
Engine oil 20W50
7. Reinstall the air filter and lock it with the clamp.
8. Install the air filter case cover and the right side cover.
CAUTION:
Make sure the element is properly seated in the air filter
case.
CAUTION:
The engine should never be run without the air filter
element installed; excessive piston and/or cylinder wear
may result.