Which natural product improve oil oidation stability

Boric acid and different borat compounds

If you are talking about the nature oxidation inhibitor which is already found on raw mineral oil its sulfur  

Base oils retain a natural ability to prevent oxidation, while hydrotreated base oils must be further fortified with antioxidants in order to maintain thermal and oxidation stability

Hi, 

You can improve the oxidation stabilty via using low linolenic canola like MONOLA.

Regards,

Devesh