Toad oil as shown on Mount Myoboku was produced from a special fountain that allowed user to more easily feel the natural energy around them. This was used in the initial stages of Senjutsu training by Jiraiya and Naruto.
However, We've seen several combo jutsu's used by Jiraiya which rely on the Toads spitting Oil (Toad Oil Bullet). Which can be lit on fire or even mixed with both Wind and Fire Jutsu. We know toads can manipulate Chakra and are mostly affiliated with Water Style.
So Is the toad oil produced naturally in the Toad's stomach's or is it something that they achieve by nature manipulation (manipulating multiple chakra natures, similar to Kekkei Genkai)?
Update: I understand "Toad Oil" based techniques utilize chakra, but the source of the oil is not explained. I have read the wiki entries and Wiki just states "converts Chakra to oil" but doesn't explain it. Is it like the Dragon Flame produced by Dragons? For an in universe example take a look at Katsuyu and her/its ability. I can see Slugs producing Slime and Katsuyu using that slime similar to that of the Mizukage's Acid Release. This is both a Nature transformation and Kekkei Genkai, while also possible naturally. Do toads naturally produce this oil, do a nature transformation, Is there any myth that attributes toad spitting oil etc. is what my question moves toward.
Additionally, for those confused about Ying-Yang techniques and Nature Transformations should go through the wikia page and read the cited information! Naruto Wikia - nature Transformation
Aside from the five elemental nature transformations, there is a second kind of nature transformation that are the source of all non-elemental techniques such as the Shadow Imitation Technique, Multi-Size Technique, medical ninjutsu, genjutsu, etc. There is Yin Release (陰遁, Inton), based on the imagination and spiritual energy of a shinobi, and Yang Release (陽遁, Yōton), based on the vitality and physical energy of a shinobi. Together, they are used to perform Yin–Yang Release (陰陽遁, In'yōton, Onmyōton).
You can't just club any random technique to be Ying/Yang.
Additional Update: Trivia section of Wikia for the Nature transformation gave an interesting lead.
Techniques such as the Toad Oil Bullet and the Poison Mist mention "converting" or "kneading" chakra into materials or substances, but have never been mentioned to use certain chakra natures.
So I think I am not the only one unsure how these techniques are performed!