is transmission problem ?

I need help!

I have VW passat GL passat 4door 4cyl 16v 2.0L 91 automatic transmission, this car I bought of oldman it look nice body. He said never driver in winter. Now, I go to repaire shop for certifi and emission test . The certifie is fine but emmission test was unneccessry . I stand these for waits that I see the mechanician he run engine to hight up to 39.7% (in screem of computer test) and them drop down 0% and he start again and again.
after them He stop test and he talk to me that can't test because the must fit something when I go back the repair shop the mechanic guy he check up then he can't find anything wrong the engine was good but he said the transmision was chang the gear litle bit slower and he check transmission oil is too much they over the mark of the stick.
The mechanic in repair shop advising change transmission oil but not warranty is problem was fit. that why I write this mess to someone have knew this problem ...please help me shoot them out.
Thaxk