I bought a house with a propane fired backup generator, I don't think the original owner knew it had oil in it...I serviced it after we took possession. The oil was black and stinky from the fumes they put in to make the propane stink is my guess. 12 years worth I estimated.
I know I have that oil in a recycling can, maybe mixed with some low mileage CB550and Massey Ferguson break in oil. I will be glad to ship it to you for a few shares of stock, Preferred Stock if you don't mind.