Yes, you can use natural oils - but ensure they are 1)PURE and 2) essential oils, not burner oils.
Teatree and Aloe Vera are two very good oils to use in aquaria - I regularly do in cases where an fish/other animal has had a scrape/nip/etc to help keep off fungus and infection, with remarkable success.
It's particularly effective on loaches, and encourages stronger regeneration of new tissue.
To use/treat a tank, mix one drop of each one per 7 UK gallons of water (or one drop for every foot) in a cup of tank water.
Stir it VERY thoroughly, and slowly add over the outflow of the filter to distribute it through the tank evenly.
Note: A severe overdose WILL make axolotl vomit, and can damage your filter's bacteria, so go steady!
However, if you have an aloe vera plant, you can snap a stem and use the goo inside straight. Premix with tankwater, and distribute as above. It's guaranteed to be free from manky chemicals then!
I wouldn't be without my plant - it really helped when Spyyk split her top lip.