Shimano GRX RX 815 Series Dealer's Manual page 154

MAINTENANCE
SHIMANO genuine mineral oil replacement
7.
Once there are no more air bubbles mixed in with the oil, set the lever
in a position where the bracket surface indicated in the figure is
parallel with the ground.
Perform adjustment by changing the angle of the handle, etc.
8.
Fill the oil funnel with oil until there are no more air bubbles mixed in
with the oil, and temporarily close the bleed nipple.
9.
Remove the syringe.
Hold the tip of the syringe tube with a waste cloth, etc. so that the oil does not scatter.
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