Diesel is straightforward, bur for other cars or bikes, the different types of gasolines can be confusing.
Benzine or gasohol?
Benzine has no ethanol, whereas gasohol is a mixture of gasoline and ethanol (gaso…ol). Here in Thailand gasohol refers not only to low-ethanol blends (E10, E20), but also to higher percentage (like E85, a mixture of 85% ethanol and 15% gasoline).
It is generally agreed that benzine gives better performance and better lubrification, but benzine is now hard to find, whereas gasohol is widepread.
91 or 95?
91 or 95 is the octane level. Basically it depends on what your car or bike was built to use. There is no harm if you fill a 91 car with 95 petrol, but it doesn’t improve the car’s performance so it’s basically a waste of money, as gasohol 95 is more expensive than gasohol 91. Avoid putting 91 in a 95 car, though. In case of doubt, especially with a bike, try both at different times and see on which one the engine runs better. If you notice that the engine is not running smoothly, it may also be a good idea to try a different gas station, as the quality is not the same everywhere.
What type of petrol/gas should I use
As said above, check your car/bike manual to know the best octane rating and ethanol percentage, or simply see if there is a sticker inside the fuel door. The choice between benzine and gasohol depends on availability, and your wanted performance/price ratio. Some people use benzine every 3 or 4 refill of gasohol.
At the gas/petrol station
All gas stations have “dek pam”, staff that is. You only have to tell them which type of gasoline you want and how much money you want to spend (or “full” for the full tank).
The sticker on this 2007 Toyota Altis says that the car runs on “Benzine, octane 91, OR gasohol E10″ (which is also 91).