IMHO, the only additive worth using is Chevron Techron. And it's often on sale or two-for-one at various auto parts stores. Use every 2,000 or 3,000 miles with a full tank of gas and refill 20 miles after the Estimated Range display reaches 0. You get an additional benefit if used with Chevron gas.
All the other additives actually do harm. For example, ones that claim to penetrate the cylinder walls must be useless and can change the chemical properties of the metal; you want lubricants on the surface. Duh!
Also, use the TRD synthetic oil filter at the next oil change; much better than the Lexus filter and available at any Toyota dealer off the shelf or special order. You should be able to get 20% off the price just by asking. The part number is: PTR43-00081
If fuel economy is your concern, the two effective things you can do is: (1) moderate use of your right foot, and (2) check and adjust tire pressures (I use 39psi).