If God intended there to be an oral Torah, why would He say not to add anything? Shouldn't He have at least mention this exception in the Torah, so we know the Talmud is divine?
Devarim 4:2
ב לֹא תֹסִפוּ, עַל-הַדָּבָר אֲשֶׁר אָנֹכִי מְצַוֶּה אֶתְכֶם, וְלֹא תִגְרְעוּ, מִמֶּנּוּ--לִשְׁמֹר, אֶת-מִצְוֹת יְהוָה אֱלֹהֵיכֶם, אֲשֶׁר אָנֹכִי, מְצַוֶּה אֶתְכֶם.
Ye shall not add unto the word which I command you, neither shall ye diminish from it, that ye may keep the commandments of the LORD your God which I command you (Mechon Mamre translation)