2001 Impala Transmission hard shift?

I was driving around doing errands today and when I left one store I noticed that the transmission was starting to shift hard. I pulled over and shut the car off for a minute, then restarted it. I checked the transmission fluid and it was fine. It was changed about 300 miles ago and had a new filter installed (Fram) and refilled with Valvoline Dexron III fluid. after restarting the car it has been fine ever since then without any more issues. I went to the auto parts store and they recommended I put in a bottle of LUCAS transmission conditioner, has anyone used this Lucas stuff before. I am no longer having an issue, should i use it anyway, and if so why? The car has 108,000 miles on it. The transmission has been serviced regularly and does not slip or anything. Just had this transient issue.