dual countershaft trans don't have synchros, they have sliding collars . there is no friction material as in a synchronized trans, thats why its called a synchronized trans. anything other than ATF or synthetic atf in a ZF 5spd will eat up synchros. I'm not a truck driver but am a truck/coach mechanic. personally I find it easier and faster to shift a dual countershaft (non synchronised) trans without the clutch. but only if I drive the truck for more than a few minutes. one big problem with medium duty synchronised truck trans is people putting them into gear initially before the vehicle is stopped. have replaced many ZF 5 and eaton 6 speed 2nd gear synchros from impatient drivers. there is a reason why so many truck come with auto transmissions now.