As title, I would like to ask when exactly did the Germany officially use Oxide Red Premier for their AFV? and what type of premiers were used beforehand? Thank you!

The German use of red oxide primer predates WW2 and was part of their normal industrial process.

From what I understand, pretty much any equipment built in Germany was given a red oxide primer coat, that's just how it was. I am unaware that any other material was used.

If something was used, it was probably long before modern industrialization and outside the purview of our hobby.

It has been used from the beginning of the industrialisation. It is called led primer and its color comes from Pb3O4. It was/is used as a base before applying the final color. Due to a lack of base material in the paint industry large amounts of equipment in Germany at the end of the war did not get their final coating. Thus turning up with the basic protection.

Thank you for your guys comments, so does that mean a Panzer I produced at the outbreak of WW2 also had a Oxide Red Premier? Thank you!

Yes.