I just ran this in 11 gallons:
Regane 2x Complete Fuel System (for up to 21 gallons)
Regane 2x Complete Fuel System High Mileage (for up to 21 gallons)
I'm half-way done with the tank, but I saw my intake valves this weekend, and they're perfectly clean, with zero carbon. The pistons, however, still have some carbon, so I'll continue to run this.
Before this, I ran 16 oz MMO in 5 gallons, which is 8x the recommended doss. It's also equivalent to running 2 Qts on a 20 gallon tank. Car ran fine. I didn't gauge gas mileage.