I have used Auto RX in my T100 and my Mitsubishi Diamante. The T100 had over 200K miles on when I performed the double auto rx cleaning and the Diamante had 140K on it. I did see a slight mileage gain with both vehicles (about 1 MPG with the truck and 2 MPG with the Diamante) and the "seat of the pants" dyno felt that it had done some good. It does work slowly and won't harm seals or gaskets and may actually help solve a small leak that you might have. The cost is reasonable for what you are getting (IMHO). If you decide to go ahead and do the cleaning and rinse with the Auto RX, be sure and follow the directions.
On the T100 when I first started using it, I couldn't tell much difference at first. Once I got to the rinse phase, I felt like something was happening, the truck had a little more power (it has the 3.0 V6) and the oil pressure at idle was actually higher. The oil pressure has remained at the higher level since I have completed the double cleaing and rinse with auto rx. I did run regular oil for the clean and rinse phases and used a Pure 1 oil filter. I have since switched the vehicles back to synthetic and the mileage gains have stayed. I will probably perform another rinse on these vehicles in about 50K miles or so. I know that they recommend that you add 3 ounces at every oil change, but I am going to wait on that one.
On the Diamante, it had a "lifter tick" in it and these vehicles are prone to have that around 100K or so. I also had some small seal leaks that are no longer there. I can honestly say that auto rx took care of both problems on the Diamante.
I have run nothing but synthetic oil in my vehicles and I was surprised at the amount of "cleaning" it actually did. I am sold on it and will use it again.
I have not tried it in the '05 4Runner yet, but when it gets 50K on it I am going to see what it can do for it.
For those that do not know what Auto-RX is, here is a link to the website:
http://www.auto-rx.com/index.html