I have a 22 gallon tank under my floor of my boat with no oil injection system. When I go to the gas station, I always add enough oil for some of the gas I know I will be putting in. If I think I will be adding 10 gallons of gas, I put enough oil in the tank first for about 6-7 gallons, then pump the gas and add the remainder of the oil after the tank is filled up. (There is always more room in the tank after filling with gas for a few ounces of oil). Then on my way to the lake, the boat on the trailer is shaking, hitting bumps, shaking and moving around enough to mix the oil & gas together real well. I never had a problem & I've been doing it this way for many years on all of my boats that had a tank built in.