I hate to break it to you guys but Synthetic is NOT REQUIRED only suggested. Read your owners manual closely
edit: For example, this is directly from the 2011 Forester Owners Manual
API classification SM or SN with the words “ENERGY CONSERVING” or “RESOURCE CONSERVING” or ILSAC GF-4 or GF-5
0W-20 synthetic oil is the required oil for optimum engine performance and protection. Conventional oil may be used if synthetic oil is unavailable.
If 0W-20 synthetic oil is not available, 5W-30 or 5W-40 conventional oil may be used if replenishment is needed but should be changed to 0W-20 synthetic oil at the next oil change.
In other words the only REQUIREMENT is API SM or SN with ILSAC GF-4 or GF-5. All the other "mays" and "shoulds" are not requirements at all.
You can run conventional oil all day long if it meets the requirements and you change it according to the interval specified. If you really want to know what is going on send your oil for analysis. Its only 20$ fricking dollars and you'll know for certain whether the conventional oil is damaging your engine.