Personally, I would consult the furnace manufacturer's documentation and the company that supplies your heating oil. There may be corrosion inhibitors, anti-gum or anti-separation compounds, etc. in the heating oil that you don't want to dilute too much.

Alternatively, advertise and sell it. Somebody will grab it at a discount for temporary heat (in a greenhouse, for example).

BTW, I recall reading about low odour kero somewhere. Probably less sulfur and other odds-and-ends. Might be more pleasant than the old-school stuff.