It does sound like you might have a little transmission slippage, which can be caused by low fluid. If you were leaking a little, possibly, it wouldn't hurt to bring the level closer to the "full" mark and see if it helps.
The Lucas brand is pretty good stuff. It would probably help with a minor seal leak, and you might take advantage of putting it in while the fluid's a little low. Otherwise, if you fill it first, you'll either have to remove some or risk over-filling the transmission.