I know we're talking as relating to the valve job, but I also used vacuum and dye to find an oiling issue on a modern era aluminum 4-valves per cyl Benelli 3-cylinder sportbike (135 hp beast) not too long ago. Brand new it was blowing blue smoke every time at startup, leaving from sitting idling at a long stoplight. Compression and leak down test were good. Was thinking bad valve seals or something related. Had to pull the head to inspect the guides and valve stems, the seals etc. Measured clearances, there was nothing wrong with the seals, I was baffled. But to further verify no oil was being drawn down the valve/guides I used dye and vacuum to see if I could draw liquid that way. Lo and behold the dye started wicking from a few guides (many of them intakes) as pressed into the alloy cylinder head! They were installed incorrectly, tearing the aluminum causing fissures where the oil could then be drawn into the port. At the time I never would have considered oil getting past the guide as pressed into the head but now I know better. And, that is something I wonder about with any alloy head with the guides pressed in. Ever see something like this??