You don't need full syn, but you do need GL-4. I know that every auto store around here has GL-4 compatible gear oils from O' Reilly's, Mobil 1, Royal Purple etc. You probably won't find any cheap, or generic GL-4 (besides O' Reilly's brand) but it shouldn't be hard to find. The GL-4 fluids are needed in order to protect the brass synchros, plain and simple. I run AMSOIL because its a product I trust and its cheaper than MOTUL.