Guide on how to refill your gas stations.
First of all you need a trailer and a oil tanker which looks like the photo given below.
(https://i.imgur.com/d7cUVSo.jpg)
Then you should move to one of the refineries whose locations and screens are given below
The first refinery which is located at flint county, It's near LS-SF tunnel.
(https://i.imgur.com/1oyiiHR.jpg)
(https://i.imgur.com/HecNyIF.jpg)
(https://i.imgur.com/FZY7xsl.png)
The 2nd refinery which is located at bone county, It's at the oil pumping job which can be found in /gps.
(https://i.imgur.com/53AgkCt.jpg)
(https://i.imgur.com/jO6wZLL.jpg)
(https://i.imgur.com/NAlbwzK.png)
After reaching at one of the refineries you must type the command /refilltrailer and the tanker will start filling it can be filled maximum with 1000 Units which cost 10$ per unit and after filling the tanker you should go to the gas station you wanna fill and type the command /refillreservoir and It'll fill the pumps of gas station, maximum of 5000 units can be filled in pumps.
Note: If there are more refineries then tell me and I'll update this topic,