Here is a very simple argument for you to consider when you are discussing the Torah. Actually, I would say that the question I am about to propose should be asked every time you are trying to make any conclusion about the Torah text.
Where it is written in the Written Torah?
For example, someone is trying to convince you that there was some other torah (oral torah) given by God to Moses. The question you must ask such person is WHERE it is written in the WRITTEN Torah? This will usually resolve most of the questions at once. In this case, you can look at Exodus 24:12, where it clearly says that Moses got ONLY Tablets of Stone with the 10 commandments on them and a scroll of the Written Torah. This is it. Written Torah never lists anything else that God gave. No other types of Torah was ever given. So here...this question provides the answer that can't be argued against.
So always remember: WHERE it is written in the WRITTEN Torah?!??