I personally wouldn't use any of that engine additive treatment crap in my motor, unless it was absolutely neccessary (like the stop smoke junk). I've heard nothing but nightmares about those engine additive stuff. I think its because they contain crap that shouldn't be in your motor in the long run, like teflon and stuff. I used this stuffon my '83 626 to prevent the smoke screen I was leaving to get worse. It really did stop the white smoke, but that was a $400 pos anyway, I could care less if it blew up.
Fuel injector cleaner (goes into the gas tank) I use every 10000km. I go to walmart and pick up a 3 pack of stp for under $10. Stuff works great and is actually recomended by most dealers. Don't expect a big gain in power or fuel economy, but it does help.
I used this engine shampoo that was a GM brand (I work for GM). For some reason, my engine seemed quieter and smoother, but I may just be tripping. Engine shampoo is usually done with a tune up, so I can't see it hurting, unless it is done incorrectly (cover ALL sensors).
Octane boost, waste of cash IMO. Unless you run regular gas, 87, and add +7 octane boost to give you 94 octane, it may be cheaper than filling up on premium at sunoco, but I haven't done the math, so I'm not sure, wouldn't bother though, unless you want to run 102 octane?
oh, walmart has most of the stuff crappy tires has, but for alot cheaper, check it out!!! And please don't buy those license plate covers with the gold chains that go around it, with matching wiper blades!!!

, just playin