Is it possible to "shoot the fuel tank" of cars?

well, the answer is yes and no.

The YES part:

• You can "shoot the fuel tank"

The NO part:

• The car will not blow up instantly as in GTA:SA(never played 4). It will instead create a gas trail that can be ignited

IGN has recently published a list of 100 Little Things in GTA 5 That Will Blow Your Mind and #64 says:

64) Shooting a car’s gas tank will create a gas trail that can then be ignited.

Check out this YouTube video on How to shoot gas tanks in GTA Online. There isn't a gas trail online, but you can still blow up cars much faster shooting them in front of the rear wheel well on the appropriate side.

The video shows the player shooting a few different types of cars in a few different spots, to illustrate the difference.