Xero

Xero is a petroleum company featured in Grand Theft Auto IV and Grand Theft Auto V.

Grand Theft Auto IV
Unlike its competitors, such as RON, Globe Oil, and Terroil, Xero does not appear to have set up any gas stations in Liberty City, although it does operate a refinery at the southwestern corner of the Acter Industrial Park in Alderney. Xero storage tanks and faded advertising of the brand can also be found in selected parts of Liberty City, and Xero sponsorship vinyls are visible on certain Bati Customs designs.

Grand Theft Auto V
The company owns several inaccessible filling stations across San Andreas. ATM's and Jerry Cans are usually found at these gas stations. There is also an industrial building on Greenwich Parkway in the Los Santos International Airport, as well as a refinery in Blaine County. In the enhanced edition of GTA V, Xero has its own blimp.

It is one of the sponsors on the livery of the Jester and Massacro racecars.

Influence
It could be based on Exxon, as both have an "x" in their names and share the same colors in their logos. Its name and logo design also appear to be similar to that of Xoomer in Grand Theft Auto: San Andreas. It might also be based on Chevron, given the fact that much like Chevron, it has refineries on the East Coast, yet no presence in gas retailing there.

Trivia

 * 'XERO' is a pun of ZERO GAS, a mockery of the oil crisis.
 * Based on the strong similiarities between their logos Xero may have Xoomer from GTA San Andreas. It is possible that the modern Xero is a rebranded Xoomer as the two chains share very strong similarities.