Not saying that this is recommended, but many Mamod stationairies have run their whole lives without a drop of oil ever finding its way into the cylinders. Fortunately saturated steam and condensed water do work to make something of a boundary layer lubricant in such situations .... but oil would definitely be better. On my Mamod steam engines I find a drop of steam oil in each cylinder lasts for several runs on any given day as it seems to froth into a film that slowly dissipates in the top of the cylinder, showing up at the mouth of cylinder through time. I can usually get an adequate amount of steam oil into the cylinder by putting a goodly drop or three on both sides of the ports with the piston at TDC, while pulling the cylinder away from the plate face against the trunnion spring, and then turning the mechanism as I release the cylinder back onto the port face, so that the piston moving down draws a bit of the oil into the cylinder.
When convenient and easily done, I do prefer to pull the piston out and put a drop of steam oil right on its head before re-inserting it back into the cylinder, being sure that I've maintained its original orientation.
I don't have any Mamod locos, but have used this same technique on TEs, SRs, SEs, SWs & SAs and always seemed to have evidence of the cylinder having been oiled after the run.
YMMV