I searched but didn't see anybody else who had posted anything about engine flushes on the SC300.
The brief back-story here is that I had a project 2002 Celica that I was trying to get the oil control rings unstuck (Common problem for this engine in these years). The flush did not fix the problem but I did notice an immediate improvement in fuel economy. I traded the '02 Celica for a '99 SC300 with a salvage title with ~250k miles.
Running the flush on the Celica had yielded positive results so I decided to try it on the SC300 knowing full well that I risked loosening gunk or possibly opening up oil leaks.
I used Walmart Motor Medic Engine Flush and added the flush to the engine oil per the instructions. I ran the motor for 5 minutes, shut it off and let it cool for 30 minutes. I repeated this 4 times, then drained and replaced the oil. (I intentionally did not flush the oil out with clean oil, just let it drain well and refilled)
Like the Celica, the results were pretty much immediate. My gas mileage had averaged slightly under 20mpg and is now closer 21 in city driving (based on calculations during fuel fill ups). If the gauges are to be believed, my highway mileage has climbed from about 26mpg to closer to 28mpg. "Seat of my pants" feel is that the engine is quieter and the car seems way more responsive, annoyingly so sometimes such that I have found myself enabling the winter mode more often for a calmer drive. I asked a friend who had driven the car when I first got it to take it for a blind test drive and he also noticed that it seemed to be more lively.
I did notice an oil smell and some oil seeping from under my valve covers but I also discovered that some of those bolts were finger tight! I have tightened them up and that seems to have resolved the problem. The engine has 250k original miles on it so I expect a little oil seepage.

On the power steering side, when I got the car, the major problem the seller informed me about was the leaking power steering pump. I had read about the problem so I understood what I was getting into. I didn't initially have the time to work on the pump so from the same walmart that I got the engine flush, I bought their house brand power steering fluid with leak stop and added that. I lost most of that fluid over the next few weeks so i topped it up again with the same fluid. This time I went almost a month before needing to add fluid. I added more fluid again and this time it lasted a few months. I finally topped up the power steering fluid in August and when last I checked, levels were still good. Your experiences may differ but this seems to have worked for me.

I'm no professional, more of spare time a hobbyist but I thought others might find my experiences useful. I have also fitted 2018 Mustang wheels, LS400 brakes, aftermarket headlights and taillights with TEMU LEDs, android auto stereo and Temu seat covers if anybody is interested.