X2 visa or short term business M type?

I have been living in PRC for two years on a full Z type working as a history teacher. I have the chance for a change of employment working in another industry however the job is not paid and is techniqually an internship for an agricultural company which is owned by a professor of Nanjing agricultural university. The study/internship is only 6 months long. Should I get a regular non resident permit business M type or should I get an X2 type? Does that cover internships? Or if I'm not being paid and will be out in sixth months should I get a tourist L type? Thanks