Here would be a prime example I derived from Buddhism:

P1. All sentient beings inherently desire to be happy or rather be free from the state of being unhappy.

P2. All sentient beings have desires that are fulfilled by the application of reason.

P3. The application of reason, to our state of affairs in the world, leads a sentient being to conclude that all desires cannot be satisfied due to the fact that we live in a world of limited resources.

Therefore, not letting passions dictate all aspects of behavior, thought processes, and action will be beneficial in reducing unhappiness, and therefore producing happiness.

There's another from good 'ol Stoicism:

P1. Things out of our control are out of our control.

P2. Things under our control are under our control.

Therefore to maintain a state of equanimity and tranquility in the world, on should focus only on things under our control as opposed to the ever changing things out of our control.

The golden rule would seem to be another example of a synthetic a priori truth derived by most ethical theories.

But, here's an even more interesting implication. Let's say for the sake of argument that ethical postulates are indeed synthetic a priori derived truths based on experience or manifest wisdom. If so, then what happens with certain accepted truths like "An eye for an eye.", which seem to be at odds with more emphatic postulates as "Do unto others as you would have them do unto you."? Do they simply exist mutually independant of each other, are disregarded or simply pushed aside?