Chevrolet Venture

I changed transmission oil, engine oil and air filter at same time. After doing this my van giving problem. When I start after long stop engine start immidietely but van does not move normally, it move very slow and also giving some jerk. When I give gas, engine give more noise but do not move, pick up is very bad. But after driving 5 km and engine temperature is up it run better, but not as better as before I did above service(oil & filter change) I went to machenics, he told me that transmission fluid is thin, thats why it is not providing enough pressure. He added some chemical to make transmission oil thik and told me it may work after driving 4-5 days. Other wise you have to fix transmission. Could you help me what atually problem is and what is the best solution for it.
November 11, 2007.

The trans fluid could be low. Or you could have the wrong type fluid in it. The filter could be improperly installed.

Dec 29, 2007.