If it doesn't help, you have something flat worn out or broke. I see the same results rather injecting it or pouring in tank. Tank method takes a little longer but a good 80 to 100 mile drive will yield results.
The BG MOA (motor oil additive) has proven it's usefulness to me as well. Built an enduro race car once from an old junker Olds that had sat in the woods for years. After carb rebuild and such, changed oil, added MOA, the engine ran but clacked badly. 15 laps in, I locked bumpers with a guy, mine came off and came back at me. It took out my radiator but I said what the hell, I'm not stopping.
After Lap 37 and been running with no water since lap 15 I was black flagged, didn't know why but came in anyway. Turns out I had an engine fire. Everything rubber was burning, hoses, plug wires even the paint and grease on the engine. Shut it down while my crew put out the fire. tryed to restart but the engine was locked tight.
We pushed it on the trailer and for sh*t and giggles I hit the starter button and the old girl fired right up. I used that same car with engine untouched except for replacing wires and hoses for two other enduro's. Maybe it was the MOA, matbe it wasn't but I have used it ever since in everything I own.