I'm thinking that with the 0.4mm size mentioned. He put a tiny slot across the top.
Going from say the oil slot you already made on the carriage, to the edge of the dovetail. So the oil dripped down into the sides as well as on the top.
If you get me.
I just drilled a hole through my gibs in the correct position so oil would squirt straight through to the tail itself.. The other tail side already has a small hole.