From 1992-2000 I was many times involved in "oil quality" discussions on BBSes, and then it continued on Internet. It is quite possible that using synthetic oils might prolong useful life of the engine. The issue is that I never use the vehicle until the end of the engine's life, I usually drive motorcycles and scooters from 0-25.000 kms and then sell them away.
Where I live, I liter of fully synthetic oil is 24 US$ upwards.
For unknown reason, one shop offered fully synthetic oil for 6 US$/liter and yesterday I bought 2 l for next changes.
It is interesting that on the average you can expect such price for mineral oils here, and semi-synthetics are around 12 US$/liter.