hard shifting - odd issue

[Pimpin Dsmstyle]

I'm not sure exactly what the kid used but he says it was GL-4 AMS oil. I went to napa before and they said that AMS doesn't offer a GL-4 specific gear lube. I'm not one to trust a worker at napa though heh.

Anyways issue is as follows. If the car sits for as little as a week, the tranny is next to impossible to shift. Once you get moving in 1st / 2nd for a short while, you can get it to go into 3rd and 4th. By the time I made it to my house ( about 2 miles) it was going in 1-4 just fine. I didn't go over 45 mph so I didn't even try 5th gear. Could it be just that he has too thick of oil in the transmission? Or maybe he didnt have enough? I've never seen a problem like this before. He told me it did this to him one time before when it sat for a month while he was driving his other car. He said on his way home ( 30 minute drive) after picking it up, the problem was completely gone. Drove normal for a few months up until it sat for a couple weeks yet again.

91 talon tsi AWD 5 speed. I originally installed the tranny for him and used some cheap napa GL-4 specific shit. The tranny shifted like butter and gave me no troubles what so ever. He felt the need to switch to a synthetic fluid of his choice going against my advice for Redline or Royal purple. I just need to know if anyone else has had this problem or if anyone has any other ideas besides the gear lube being too thick / wrong weight / no MT-90 to lighten it up.. Any ideas are appreciated.