changing diathermic oil?

I have a Lakewood Mod 7000/A elec. oil filled radiator. It said permanently sealed but is leaking at the element insert hole. I opened it and want to top it off or change the oil all together since it looks like my engine oil when it needs changing. I know a replacement would be fairly inexpensive but I enjoy depriving manufacturers of built in obsolesence especially when they say permanently sealed and it leaks and avoiding adding more to the land fill. I've searched for the availability of small quantities but not having luck. Where is this type of oil normally sold in the USA?