There is an update that adds AI Special Infected to Scavenge, so if you haven't updated yet, don't do it because it will make this harder to get. Only special infected can make you drop a gas can.
Some tips: Use adrenaline, it makes you run faster and fill up the gas tank faster. Throw as many gas cans down to the ground as possible before you pour them all in, but make sure you have enough time to reach the gas cans from where you are. Each gas can adds 20 seconds to the time, even when you go into overtime, so don't worry if you go into overtime as long as you are holding a gas can, you should be fine. Try not to shoot the gas cans as they will explode, often destroying the can next to them, and you only have 16 to begin with. Using melee weapons is recommended to avoid accidentally shooting the gas cans.
It is best if you do it on Dead Centre since it is a smaller map and the cans are therefore closer together and more easily reached. This means you can throw more towards the centre and reach them quickly to fill the gas tank.