A ฿60-100k car in Thailand will likely be older and more well-used than you anticipate. It could well be a money pit. You also need to factor in the cost of insurance and the hassle of the annual vehicle inspection. A rental will be far less stressful. I've driven tens of thousands of miles in Thailand without a problem (so far) but it is definitely more dangerous than Western countries. You need to stay alert.
A Honda car would typically call for a 75W-85 GL4 gear oil. This has a similar viscosity to a 5W-20 engine oil. Chev and Isuzu have specified 15W40 engine oils in some of their SUV's for a long time. Both engine and gear oils have extreme pressure additives to give about the same Timken rating. Engine oils have detergents and dispersants that are not required in gear oils. Gear oils have higher levels of anti-foaming agents. The main difference is that gear oils have friction modifiers to make a oil a bit less slippery and hence allow the synchros to grab cleanly. Using engine oil w