Does transmission fluid make a difference?

I had to replace my radiator recently and lost some tranny fluid, so I decided it was time for a flush. Changed to AMSOIL full synthetic ATF, and now I have what seems like an engine RPM dependent driveline shudder.

I had this transmission installed about 60,000 miles ago, and had not changed the fluid, but it has performed perfectly the whole time. The guy who installed the tranny for me (it was January - brr!) said he used a Pennzoil synthetic blend ATF.

I drained the pan and got better than 3 qts. of fluid, bright red and clean. Refilled, drove on it for a week, and did the same thing again. About this time I started feeling a driveline vibration and harshness. I have checked the u-joints, which seem smooth and tight, the center support bearing, which has no noticeable play, and the tranny tailshaft bushing, which has a tiny bit of play, but no more than the other AW-7x transmissions I have access to (and no leaks).

So, I am beginning to wonder if the synthetic fluid is causing torque converter chatter, and if going back to a less space-age fluid would help. The problem seem much better this week, but I wonder if it is because the temperature is about 18 degrees lower?

Side rant: I went to NAPA looking for that Pennzoil ATF and they did not have it. But the ditsy woman there told me that one should NEVER change transmission fluid after it is over 30,000 miles, and that a different fluid couldn't possibly help, but if I bought some snake-oil additives, I might fix the problem. I got out of there in a hurry so I didn't say something nasty.

Thoughts?