Always refuel when passing a gas station. Every small town has one but to stay save always refuel. Some remote gas stations might appear closed or the person in charge gone for the day. Look for a telephone number or a local nearby to ask and most of the time the person in charge will quickly appear.
Take Octane Booster and 2T oil from Ulaanbaatar. The BETA Xtrainer has a 2T oil tank that you can fill up once in a while after several tank loads.