Visa for retired univ. prof., invited to teach a short course in university summer program

Three Chinese universities have invited me to teach short courses (one to two weeks) in their summer programs. They pay expenses plus an hourly fee for the teaching. All three universities insist that this can be done on an F type entry permit, and that Z types are only for foreign lecturers working for six months or more. Is this true, or are they simply avoiding the paperwork and bureaucracy a Z-type application would require?