Not always true.

Using a different type of oil does not automatically extend oil change intervals unless the engine can take advantage of the properties of the synthetic oil. If the oil gets dirty, it gets dirty, and must be changed. The idea that "fully synthetic" automatically means longer intervals is one that many ZZ-equipped Corolla owners have discovered to be, sadly, not applicable in their case. (look up Toyota sludge issues)

Many CRDis already require fully synthetic oil, but still use 5,000 km intervals for local conditions. This is because Philippine conditions are considered harsh. This is not to say that the oil is useless by 5,000 kilometers, but unless you have access to an analyzer, do you want to take the risk?