Six months and it’s still healing? That oil is likely doing nothing. Given a proper plane of nutrition and treatment for infection, yes, I would expect most wounds to close up within 6 months, even if there’s infection or proud flesh at the start.
If I was going to attribute anything to “oils,” it would be that a lot of them are actually irritants and should not be put on open wounds or mucus membranes unless you want to scorch a layer off. Tea tree oil is a prime example of a product people think is all natural and magical, but it can be extremely irritating, especially in high concentrations. That might help if there was proud flesh to treat. Still, there are far better treatments than mystery oils and I would caution against letting anyone put essential oils on a horse, especially one other people handle, which might expose multiple people to these ingredients. It is amazing how many people are allergic to various essential oils. If it is the oil “helping” reduce the proud flesh, I would stop using it, because something that works on proud flesh will not be good choice for healthy tissue once the proud flesh is resolved. You have two totally different processes. Treating proud flesh requires destruction. Closing a wound requires healing. The oils might be why it’s still open at 6 months and a little benign neglect or a moisturizing layer (honey, etc.) might be what it actually needs.
I think what we have here is a failure to understand the “science.” It is not impossible to treat proud flesh or scar tissue. It does not take multiple surgeries in many, if not most cases. Wounds over high motion areas heal all the time. In fact, they almost ALWAYS heal without resorting to any of the above. They are just slow and require patience and care. The last proud flesh case I treated over a joint resolved with daily steroid cream for a week and from there it resolved via benign neglect (and not letting dirt get in there.)
Honestly, I’m very perplexed that your vet thought these wouldn’t heal and you should just accept an open wound. Either there’s been a misunderstanding on how far the rescue is willing to go or your vet is uninterested in doing anything further. A non-healing wound is either a sign of continued infection, a foreign object, or a call for further action. Horses live on dirt - it will eventually get infected again. I mean, punch biopsy skin grafts are lots of fun and easy! Any vet who’s not excited about that is missing out! I’ve got friends working with leeches. Friends using tilapia skins. There are so many fascinating options for closing ugly wounds. Yes, if you have exhausted the available options, this would be a very different conversation, but to just tell you to expect it not to heal and walk away from it? Odd.
Just as a note: that joint is not “basically the wrist” - the “wrist” of a horse is what we call the “knee.”
Also, I know two bonafide shamans who went off and studied for 5 years in one case and 7 in the other. Neither one uses essential oils. In 2020, when I hear “essential oils” and someone tells me they’re putting them on living tissue, I run away. Essential oils should be handled with care and are not for “potions.” I do not know of a shamanic practice that uses them. I do know lots of twenty-something-year-olds in MLMs think they cure cancer.