FarmVille Freak Eva16 and several others have reported receiving new fuel popups when fertilizing neighbors crops. When fertilizing farms players receive free fuel, but has not been a popup to share with others, the fuel is added to your overall amount.
When fertilizing a neighbors farm, players have now received popups asking them to share free fuel directly with the neighbor whose farm they were fertilizing.

FarmVille Freak Eva16's New Free Fuel Popup
After clicking the share option, a post goes directly to the farmers Facebook profile wall- not homefeed. The neighbor will be able to redeem one large can of fuel from clicking on the post.

FarmVille Freak Dave_NC's New Fuel Facebook Profile Wall Post
Note there are now 6 ways to get fuel:
1. Buying fuel for $FV in the market- You can purchase fuel in the FarmVille Market, 1 Refill for 3$FV, 10 Refills for 25$FV, 20 Refills for 40$FV, 30 Refills for 55$FV and 50 Refills for 65$FV
2. Free Fuel from plowing- Farmers randomly receive popups to share free fuel when plowing their farms. You can collect 1 large can and 9 small cans from neighbors per day.
3. Fertilizing neighbors crops- You can find free fuel when fertilizing your neighbors crops, this is at random and will be added to your overall fuel amount.
4. Completed Collection redemption reward- When trading in a completed collection you receive 5 Refills of fuel as a reward.
5. Farmers Market- When a neighbor shops at your Farmers Market stall, you have the option to redeem 15 units of fuel (this is one of 3 options).
5. Mystery Egg Prize- When opening Mystery Eggs you can find various amounts of fuel depending on the color egg. The current prizes available are 1/5 Refill, 1 Refill, 10 Refill and 20 Refills.
*6. When a neighbor fertilizes your crops and directly shares with you, as mentioned above. This may be in limited release.