I am not familiar with the 504, it would seem to me that if you have oil up to the rocker shaft and one is still squeaking its probably a clogged rocker shaft. They sometimes fill up with sludge and plug the oil holes to the rocker arms.
On the U series and 206 series rocker shafts you could take the plugs out of the ends, disassemble and clean everything and they would go back to oiling. (Putting the rockers, stands and spacers back in the same order.) I am not familiar with the 504 so I don't know if they are similar or not.