When I built my Cantenna, back in the 80s, the documentation said you could use mineral oil. You just weren't supposed to key the transmitter as long as you could with transformer oil.
I went to the power company and bought a gallon of new transformer oil from them.
Twenty eight years later I'm still using that oil in that same Cantenna.
I recommend you find a source for transformer oil but in a pinch mineral oil will be just fine.
If you are lucky they might even give you a gallon. I found out where one of the local electric company's crew shop was, went there and simply asked. I told them I was a local amateur operator and what I wanted the oil for. The foreman asked for and made a copy of my pocket license and I wrote out and and signed a notation of the oils intended use and they hooked me up.
Hope this helps!