In addition to what Patman is saying, after swithing to synthetic I would do a normal oil change (3 to 5k Miles) and later extend that to 8k miles or more. The reason, synthetics often disolve a lot of the sludge that sometimes forms in engines that use conventional oil. This sludge could overwhelm the filtering capacity of your oil filter. If you always drive for longer periods and the engine really gets to its full operating temperature I would go for 10k miles changes. However, at least for the first time I would do an oil analysis. That will give a good idea of how long you can go between oil changes.