Nobody here has a factory service manual? I have one from 1969 and it shows ATF for cold to mild climates. I have used Valvoline Dexron ATF in the 833 in the FrankenDuster for years. ZERO troubles. The ATF+4 is synthetic and may have a lower coefficient of friction, which would certainly wreak havoc with the synchronizers. They need some friction to work properly. It is this need of slight friction that Hypoid type gear oil is also NOT recommended for manual transmissions.