Grooving lifter bores

The placement of the grooves coresponds to the area of the lifter face that contacts the cam lobe first. On a clockwise rotation the intake lobe hits the lifter on its "interior" side. On the exhaust lobe contacts on the "exterior" side.
To be honest - I don't see this doing much more than bleeding out oil pressure because both lobes are splash fed more from the crank than anything else and the lifter is rotating, tossing the oil out from the mating surface anyway. If you feel for some reason you need more - use the EDM lifters like RRR says. They feed oil from the center so it gets spun out to the mating surface area.