The number of gas stations that sell pure gasoline in Thailand is getting lower every year. In most cases, they sell gasohol at the gas stations. Gasohol is a mixture of gasoline and ethyl alcohol. Octane number rating of gasohol is 95 and 91. There are also biofuel types E20 (20% of ethyl alcohol) and E85 (85% of ethyl alcohol), but only cars with engine modifications that are required to run biofuels use these options.
Absolutely every motor vehicle produced in Thailand during last years is designed for gasohol 91 and gasohol 95.
How a gasohol 91 or 95 pump at the gas station looks like:
Along the road in a countryside, you can find barrel pumps
Small self-service pumps that never give change.
And rum bottles with gasoline.
In most cases, the last three options provide you with gasohol 91, because the target audience of these gas stations are motorcyclists and, as I have mentioned earlier, motorcyclists in Thailand use gasohol 91. This means that vendors follow this category of drivers and sell exactly the gasohol 91 and not anything else.
If you are not sure what gasoline to use for your motorcycle, it would be better to check it with the staff during signing a rental contract at the office.