This is largely a consequence of the type of burner used. In my opinion you will NOT get happy results out of firing high volatiles admixed with heavy oil in, say, a von Boden-Ingles burner!

Something Dave has been investigating is centrifugal separation of the waste oil "feedstock." This immediately gets rid of any 'slugging' problem with antifreeze or water, or problems with uneven BTU release. Equipment to separate up to several hundred gallons per hour are commercially available at what I consider reasonable cost (one-time investment of less than $1000, excluding tanks), and the fundamental design of the separators is not beyond a decent museum mechanic's skill. In my opinion, it might be possible to add a 'fractionation' step to the purification achieved by such a system (consisting of controlled heating and agitation with the vapor being removed under partial vacuum and condensed in an exchanger against cold water) to get rid of any volatiles, methylene chloride, etc. that might have gotten mixed in 'by accident'.

When this system is in use, it may pay to prewash the feedstock with water and perhaps chemical treatments to remove some of the detergents and contaminants, including those from additives or aftertreatments like Slick 50. Some mix of pre- and post-filtering can also be used -- this could be considerably cruder and cheaper than what's required to purify the oil for re-use as a lubricant!

Note that a good working relationship with a fairly small number of local shop businesses or stores with 'automotive facilities' like the larger Wal-Marts will likely produce as large a source of oil -- to be processed at a steady number of gph -- as a given operation could set up tankage for. I do not know if museum operations would need some form of licensing or EPA certification to engage in this operation,

My personal preference would be to see if those plastic ag tanks with metal frame surround -- I see many of them apparently abandoned around my area, and I suspect they can be 'had' cheaply for the asking -- can be used for oil storage. These could be easily elevated to a 'stand' by a simple crane, or front-end loader, or hooked up to a pump and hose for fueling. (They also fit nicely in a pickup truck for remote access...)

Something I would like to address a bit peripherally, with respect to HudsonL's post:

1) Getting the coal into the tender bunker relatively unbroken is as important now as it was to the AAR in the late '40s. I'd be interested in reading what methods people in this community use when setting up their storage piles, and then when loading, to minimize the coal's breaking up into fines and dust.

2) A very substantial item of importance with watering is to avoid introducing additional oxygen when possible. The old-fashioned methods that put the spout or hose above the tank hatch and just 'pour it in' add much more oxygen for the boiler chemistry to knock down. In my opinion it might well 'pay' to try extending a feed to near the bottom of the tank, and introduce the water below the surface to reduce splashing and atmosphere contact to a minimum -- when pumping from a 'town' system this might involve nothing more than an 8' length of vertical pipe to near the bottom of the cistern with a standard firehose standpipe coupling sealed on top, with a level float or gauge located alongside (you'd insert the gauge rod when taking a level, with the end resting on the appropriate part of the float, and take it out for storage afterward)

Then close up the tank relatively tightly, and use some sort of cheap gas blanketing -- this is a useful place to use 'remediated' carbon dioxide from "carbon-harvesting", imho -- and the usual chemicals in the tank to knock down the dissolved oxygen. (Note that I've been told that the tannin in the Porta-McMahon system is capable of dealing with 'normal' levels of oxygen in feedwater, so this may be overkill... but in the absence of effective treatment, or when high tannin can't be sustained because of priming or carryover issues, it might be useful.)

Water Utilities are terrified of the possibility of back-flow contaminating their water systems. It got so that I took pictures along when I arranged for using hydrants, of our fire hose and L shaped filler pipe, to explain that there was
no chance of back-flow.

-Hudson

Good point about backflow prevention. The importance becomes more extreme when tender water 'dosing' a la Porta is in effect, because the contents of the cistern may have high proportions of 'interesting' chemicals.

I am not certain what the appropriate size for 'firehose' filling would be, but it would probably be something like a Dixon BFP-200, mounted horizontally (the design has to be) adjacent to the fill pipe to the cistern bottom. Assuming you are always going to be filling the thing from a firehose, a simple double nipple (DMN2025F) is all that's needed at the input; I'd fit this with some kind of screen, but that's peanuts to make. (The BFP-200 is what Dixon recommends for backflow provision to hydrants, so the relatively small 2" nominal diameter might not be as restrictive as I'd think at first glance.)

Astoundingly to me, Amazon claims to sell this for just over $700, and ship it to you for $9 (it weighs about 50 lb). The DMN2025F is another $27.80 and will ship free if bought along with the valve. This is not small change -- but it only has to be done once; in cases of advanced skin-flintedness, it would be simple to 'gin up a simple union connection and bracket that would allow the unit to be shared between locomotives...