Nice looking Stuart 10 there, with inline displacement oiler and all.
The 504 is a much wanted boiler, of high quality and good steam productivity, you won´t regret that purchase.
Regarding the inline oiler, you simply unscrew the lid, and fill the oiler with steam oil up to the connecting pipe, before you run it.
No need for it, if you run the engine on air, as it doesn´t function without water/condensate.
As you run your engine, steam passing by through the steamline, will bring along some condensate, which slowly will fill the oiler.
Water being heavier than oil, will settle at the bottom of the oiler, and thereby lift the oil out of there, into the steamline....dunno if that made sense?
The valve at the bottom of your sightglas, is a draincock to empty the boiler. If opened while there´s still a bit of pressure left, it´ll blow out most water that way.
I assume the opposite attachment is a blanked off connector, to which you could mount a feedwaterpump, and fill your boiler under pressure.
AS far as oilers go, you should get a larger one, to handle the fat steamoil used for the areas in contact with steam (like the displacement oiler on the Stuart 10), and a dedicated presicion/needle oiler for thin automotive oil like a higher grade Mobil1, to lubricate all moving parts and bearings not in contact with steam.
But far as specific oils and alcohol goes, i think you´re better adviced from others, as i don´t know the best/cheapest approach in the US.