air pump will work well with 40 weight oil that is room temp. . depending on which engine you have in the loco you are getting, the lube oil capacity can be anywhere between 200 gallons and 400 gallons. .
at work, when we pump oils out of drums [gear oil, compressor oil. . ] we use
graco air pumps... we feed them shop pressure or regulated pressure depending on what we are pumping [gear oil full pressure 90psi, compressor oil about 1/2 shop pressure]
our service trailers have lube oil and cooling water tanks on board, as we use diaphragm pumps to pump the oil or water when we service the loco's at the outposts
i know both types are expensive, but the diaphragm pump is easy to rebuild when the time comes. they can draw from much lower than the level the pump is at [we use a 2" one in our water treatment plant, and it runs basically nonstop and usually lasts 12 months before it fails [housing errodes] and draws up 20']
we also use a 1. 5" version to pump the treated cooling water through my shop for filling the locomotives up. that one lasts about a year too, but it gets plugged up with the borate nitrate we use as the water treatment
the lube oil at my shop is pumped by electric pumps [constent running motors, that unload the pump when no flow is needed] but we have much more than 5 barrels on hand [the tank in the tankfarm is about 15,000 gallons or so - filled by tanker trucks]