In real world use, 2 of the most significant aspects of fully synthetic oil is the high temperature stability and the potential long service life. This means the 10 or 15k miles oil change intervals common with many cars now.
Some of this is to do with the lifetime of the additives which regular mineral or even semi-synthetic oils need to avoid the viscosity changing too much.
The high temperature bit really applies to bore surfaces etc.
Either way, an engine which doesn't expose the oil to very high temps (e.g. above 130C sump temp or 160C bore surfaces) or gets oil changed frequently, doesn't really make full use of the capabilities of synthetic oils.
There are genuine benefits from using semi-synthetics rather than straight mineral oils in that the properties come from the oil molecules themselves without the reliance on so much additive use (which tend to degrade faster). Indeed certain viscosity range mineral multigrades have been severely criticised in the auto industry due to the excessive viscosity improver additives and problems that leads to.
FWIW, I use Castrol GPS 10W40 in my bikes, but I wouldn't say it's any better or worse than any other good brand bike (JASO MA) oil. I find it performs well in my engines.
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