Fully synthetic can cause problems on some engines. A mate of mine used to work at one of the oil research labs on Canvey Island and was involved in a project that looked at this very issue. There were some reports of excessive "glazing" of the bores when using fully synthetic oils, particularly if the oil was changed frequently. This has been noted in some motorsport categories where the cars dont do much mileage between changes and the result has sometimes been premature bore wear - blue smoke from the exhaust under acceleration and sometimes when engine is idling and is running at high temp.
For what its worth, Ive often been told that if I use fully synth. then dont bother with more frequent oil changes than recommended - stick to 12k on the 172. Alternatively, use semi-synth and change at 6k.