The thicker the oil the better ! Anything will do, just a nice thick gloopy oil, and just fill the oiler cups before each ride. Unless you have modern seals at the bottom of the spherical bushes it will seep out quite quickly, and you will realise that the so-called ‘water deflector’ on the bottom edge of the brake plate is really an oil deflector !! So many oiler cups have been wrongly replaced with grease nipples over the years, and using grease means that lubrication never really gets to the bottom bushes where it is really needed. Over many years the grease turns to a solid waxy consistency and fills the tubes, stopping anything going down from the oiler cups/grease nipples.