As the title says, Car Nitro that you can carry around in your inventory, since in GTA it's depicted as something that's small enough to store. Mechanics or the NPC mechanics at gas stations could sell this to you. If there are concerns about it, it could be limited to only the 2X or 5X Nitro and a fair limit to how much you can carry, but otherwise the 10X Nitro would be included.
Once you have it, you can simply open up your /inv while in a car to apply the Nitro.
This can be helpful because: you can install Nitro yourself into any car you might have (especially for cars you just need to drive for missions), and also refill the Nitro in your car quickly. Saves a trip to the NPC mechanic.
