Never noticed any sloshing fuel......... This is the way I use the auxilliary fuel tank. The main tank holds 6 gallons, and the auxilliary tank holds 5 gallons (advertised as 4 gallons). When the low fuel indicator comes on, I transfer fuel from the auxilliary fuel tank until the fuel gauge shows full. Then, I turn the pump off. IMPORTANT NOT TO FORGET THE PUMP ON, OR GAS WILL OVERFLOW ALL OVER YOUR TANK! The auxilliary fuel tank appears to have an irregular shape inside, which may provide some baffling. Never hear or feel anything from the rear.