On a regular basis, Nothing.
Once a year, for the last 1k of an OCI I will use 16oz of MMO.
This has proved itself to me to do some cleaning, but I see no reason to use it 'all the time'.
In the Gas, once a year, I use 4oz of MMO or 4oz or Canadian Tire Fuel injector Cleaner for 5 consecutive tanks. Again, these products have shown themselves to have a positive effect, but don't (to me) to be required on an ongoing basis.
In the distant past I have used STP in a chronic oil burner, It proberbly extended the engine's life 6 months at a time when I had few other options.