Also, whenever you try new things, don't expect it to go over immediately.  when I changed the library cats to EP, I put a bowl out for two weeks along with their food.  The would sample a little here and there and eventually, they preferred the ep.  Would work for oil too.

If your vet told you that Panacur kills all parasites, you need to get a new vet.  It does not kill tapeworms and tapes don't always show up in stool samples.  A vet however, should know that Panacur doesn't do anything for tapes. Praziquantel is the only stuff that does.  Droncit is a common brand of Praziquantel.

Thirty minutes after cleaning up moving filaments in the litter box, my cat passed a stool sample for tapes.  My vet said that that was very common because tapes don't always show up in lab tests. 

Not that your cats have tapes but Panacur does not work on all parasites.

I did say that "not that your cat has tapes."  I really don't have a clue what your cat has. I just understand the misery of the mess. My cats never lost weight with tapes.  They didn't vomit either.  Except once, Magellan actually vomited the things up.  Tapes gave my cats horrible bad breath and horrible, smelly diarrhea.  At some point, there will be those half inch wiggling thingees in the stool. They would get reinfected every 30 days.

Having said that, I do know from participating from lots of boards that your problem is not uncommon. I don't know if you have tried New Balance cat food but it has few ingredients and is popular with people who have animals with food intolerances.  It helps

I am sorry. I used to have a good mind and will always miss it.  Yes, it is Natural Balance. 

Purina One was unusually successful as well with Magellan and his hairball issues.  I used the Hairball formula.  I have had some very healthy cats on purina One even though I detest the ingredients.  All my other sensitive stomach cats did well on it too.

Update:  It turns out that this vet (I've been taking my kitty friend to a large veterinary hospital where there are lots of vets, so she hasn't seen this one every time) spent a lot of time explaining everything to me once the tests were in.  She says the likely cause is irritable bowel and/or food allergies, and stool tests were negative--but that does leave a possibility of some less common kinds of parasites, most of which there's no treatment for but cats can eventually get rid of on their own, and cats with irritable bowel and food allergies are more susceptible to them.  Again, it comes down to diet.  We're putting her on Royal Canin duck and pea formula--checking the ingredients, it looks like it's similar to Natural Balance but higher in meat; Natural Balance lists peas as its first ingredient and Royal Canin puts duck before peas.  And kitty gets Pepcid and probiotics added to her food, to help her system balance.

I hope there may finally be some light at the end of the tunnel!