The best engine oils available in Thailand are from Liqui Moly. These lubricants are made in Germany and are the most popular brand there. They offer a wide range of lubricants specific to each car type and are approved by the car manufacturers, which is important for your warranty.
Liqui Moly lubricants last longer and can be used up to 15,000km. You can find the entire product range and get the engine oil changed for free at MyCarRepair quick service workshop (T: 076-600069) across from Thai Watsadu close to the Heroines Monument.
The team speaks English and can give you the right advice.