Our 1983 300D with about 165,000 miles has been slipping for a few years between the 3rd and 4th gear shifts. Last summer we had the tranny fluid changed and it seemed to work much better for a few months. It now seems to be slipping more and our mechanic says vacuum and other settings are OK. Would replacing AT fluid with synthetic help eliminate the slipping and if so, what brand/type should we use? We won't be keeping the car much longer but if a fluid change to synthetic could help, I would do it.