I too have a waste oil furnace that burns anthing from #2 HHO/Diesel to 50SAE. I don't generate enough oil here to heat the shop all winter, but depend on peope giving me oil, or if enough, I will pick it up.
Might be an idea to post an ad on Craigslist, or if you are on FB, post it on Marketplace for "free oil". There may be more owners of a WOF in your area than you realize.
The key is to keep contaminants out of it such as water, anti-freeze, etc. If you store it in jugs you just emptied, you can store it outside, but if in a barrel, keep it inside, or at least the top covered well. If water collects on top of a barrel, heat from the sun will expand it during the day, then when it cools in the evening, it will cause a vacuum, and pull water in around the bungs, no matter how tight.
I just picked up 3 barrels of used oil from a local tractor dealership yesterday, and glad to get it. Most of it is 80% hydraulic oil, and 20% motor oil, with people bringing tractors in for seasonal oil changes, and/or other repairs where they have to drain the hydraulic system. This will fill my furnace's tank, plus a 60 gallon air compressor tank I have a pre-filer set up on it, to filter the oil, before going into the furnace tank. Doing this extra step prevents a mid-winter service to clean the pre-heater, and burner unit gun, and nose cone. No pump involved, I just hook the shopaircompressor up to it, and push the oil through the filter system, which consists a 200 mesh washable filter, then through a HHO felt filter. The HHO filter is good for 500-600 gallons, depending on the oil. Diesel HD truck'sand farm tractors seems to be the dirtiest, but adding this filter really has helped.