I switched to Amsoil synthetic at around 90k. I first changed what was in the pan and ran it that way for a few thousand miles.
After that I did a full flush by using the tranny pump to push out the old while adding new at the same time. I am not sure if 100% of the old came out but it was probably pretty close. I changed what was in the pan again since that also.
Car now has 105k and trans still works great.
