A cat's skin is much more sensitive than human skin so please consider this when using anything on them that stings.
Also, I have read in numerous places that essential oils, even on flea collars, can be lethal to cats. Putting motor oil, which is a carcinogen (as is petroleum jelly), on an animal that cleans itself with its mouth, I would think that could be dangerous. While I still wouldn't put it on a horse, they are different than cats and dogs and will not likely ingest it.
Now onto ringworm:
Both my cat and I are sick - we were both exposed to large doses of a pesticide, in an enclosed area, and now both of our immune systems are having issues. Add to that, we both caught ringworm, from a stray kitten, on a cross country trip. I have found, being that I have a compromised immune system (isn't working at all) that getting rid of ringworm has been difficult as I am having to get to the underlying causes in order to really get rid of it (in chinese medicine - dampness). Ringworm is related to all the things you hear about that start with the word tinea - such as athlete's foot, nail fungus, scalp itch, jock itch (pardon my bluntness). And as any one who has had any of these knows, it's really difficult to treat. So even when you treat the raised sore, you have often not gotten rid of it (I think it is systemic but I might be wrong). So, if you are ready to take it out completely, you will have to address not only the lesions but clothing, bedding, floors, etc. You may not show sores but you may have it (itchy scalp? White soft buildup? Crusty ears? Itching anywhere?).
So even tho it is caustic, I use bleach if I have to wash floors. And where I can't, I use plain old salt. For my kitty's ears, I took a damp washcloth soaked in a high saline solution (kosher sea salt) and just applied it directly to the ringworm. This kills it within a short period of time (10 minutes should be plenty). It stings while the fungus is alive and stops when it is dead. I have found one application to be sufficient if you treat the other areas. For humans, swimming in the ocean is a way to alleviate it on the skin, scalp, etc. What you have to be careful of is when you put your clothes back on - or your kitty goes and lays down where he usually does - he and you are probably reinfecting yourselves.
When not near the ocean, I like to take kosher sea salt baths - 1.5 lbs in bath water (this is a very heavy concentration but I like it). After I am done with the bath - I throw my clothes in the water and soak them before washing them. Unlike when I swim in the ocean which leaves a residue and dries out my hair, the kosher salt leaves my hair really wonderful. I don't use shampoo on those days and my hair has been stripped of all the impurities that build up (yeast/fungus in your hair makes it slightly gooey).
Supposedly dry cleaning kills it as well.
Now this is a lot of work. I'm pretty sick so its been hard clearing it and you might not be as susceptible, but your cat or dog who is scratching and you don't see anything, or their ears are red and sensitive might be harboring more of the fungus.
I once went to a health food store and the woman behind the counter told me that she had had it a while back and had successfully treated it. I did not have it at the time. I caught it again from her - and it was a particularly virulent strain - it took a lot to cleanse that one out. She had simply suppressed the lesions.
The other thing to do is treat yourself and animal on a nutritional basis. Good food, pure water, etc is a great help. Fungus loves sugar (see candida - it's a fungus as well), so out went all the junk.
I am being treated by a Traditional Chinese Medicine practitioner and he is helping me on a constitutional level so I don't self prescribe - I let him do his work. But I do take the baths whenever I know i've come into contact with a fungus (I am sens to it now and know - I sometimes pick it up outdoors or on my own keyboard). My cat gets homeopathic remedies which clear it out of his system. Berberis is a great acute for ringworm. In a pinch (and not in true homeopathic prescribing, ringworm is a remedy as well as well that can be ordered from Helios, and that also knocks it out).
My kitty lets me put the damp cloth on his sensitive ears without much of a fuss and its a good holdover until the remedy kicks in (can take up to 4 weeks or so to be fully cleared and may need to be repeated every 3 months depending on how deep a level it affected the animal). You'll know. He still gets it but it is getting weaker and weaker in him.
I am also going to start him on hydrogen peroxide therapy as well after reading this wonderful site!
Sorry this isn't incredibly simple but it works! I hope this can help someone.