Some synthetic oils, like Mobil 1 Extended Performance, are advertising oil changes every year or 15,000 miles, whichever comes first. There are two concerns, however, with leaving oil in service too long- especially if you do a lot of short trips. One is acid build up due to contamination from combustion. Water and other things get in the oil and these form acid compounds. Whenever the engine is shut off the oil stratifies and a layer (or more) of acid forms.While there is no real concern about the acid eating through the block or oil pan or what have you anywhere the oil is in contact with a gasket, o-ring, seal there is acid at work on that item. The other concern is the depletion of the oil's additive package. Some synthetic oils are not very seal friendly. An additive ester is added which helps counter the negative effects of the base oil on the seal. But the "life" of this ester additive may not last beyond 6 months. If the oil is not replaced with fresh oil, the seals could start to deteriorate and the engine could develop oil leaks that could be very expensive to address. Something to think about.