Rot Gut Engine Upgrade
From FOnline: 2238 Wiki
The Rot Gut Engine Upgrade is an option available from T-Ray that allows you to fuel your car with Rot Gut in addition to SEC and MFC. When standard fuel options are limited, it's convenient to have this fuel option as Rot Gut can easily be made with fruit, wood, and a still.
T-Ray charges caps for this option and the price varies.