First
Make sure to lay the high-pressure pump flat, otherwise it will be difficult to measure the amount of oil.

then
Add 15W-40 diesel engine oil. Fill the oil to half the position of the high-pressure pump oil window, which has a small red dot in the middle. Fill the oil to the position of the red dot.

The position of the oil window
Basically, the oil window of all high-pressure pumps is located behind or on the side of the pump