This is a hydraulic lifter slant, but the oil passages are the same as on a mechanical lifter slant:
Oil travels up a gallery on the back of the block from the rear cam journal, into a small passage in the back of the head, and then to a small chamber underneath the rearmost rocker shaft bolt. The oil then travels up into the rocker shaft, out the holes in the rocker shaft and into the rocker arms, then (on a mechanical lifter motor) out the passage on the tips of the rocker arms, onto the valve stem, then the oil runs down into the lifter gallery and from there back into the oil pan. The only major difference between the oiling systems in a hydraulic lifter and mechanical lifter slant six is on the hydraulic lifter slant the oil also travels to a hole on the pushrod side of the rocker arms, down the hollow pushrods, and into the hydraulic lifters.
Only way to clean the passages is to (a) get the head and block hot tanked and/or (b) pull the head and block and run a pipe cleaner through the passages.