The issue is, regardless of FS or Mineral, all OCI must be the same. There is NO such things as Synthetic can last longer rubbish.
If Mineral have to changed at 5K, then Synthetic must also be changed at 5K. If Synthetic can change at 10K, Mineral can also change at 10K. Period.
This is an interesting topic to European 'outsider' as I am.
Here in the UK, Proton stipulate service/oil change every 14,000-15,000 Km, (9,000 miles to us) My P1 SC uses medium quality 10W-40 Semi-Synthetic oil, (10W-40 seems to be most garage's bulk buy 'one size fits all engines' option, lol) and although not good enough for me, (I use 5W-40 Fully Syn.) no-one seems to have any problem with poor running or engine failures by following the official P1 advice.
A few car/van makers have recommend oil change intervals as long as 24,000Km, (15,000 miles) depending on type of use, (some Renault, VAG) - although most experts don't agree with so long an interval. 19,000Km/once a year, (12,000 miles) is the norm for most cars here and has been for many years.
Our lower average tempratures may well be an issue, as oil loses it's 'top-end' viscosity performance in time - a typical 20W-50 is reduced to an effective 20W-30 with extended use.
Perhaps more likely the quality of oil is better regulated and so more consistant over here? (it is rare to find any 'fake' oil/parts in a normal Euro SC)
The point about a Synthetics additive/OCI package degrading as fast as on a Mineral oil may be true with some oil companies products - but even if it is, that's only part of the issue.
Full Synthetics big advantage, (even more so in cool climates) is their greater flow/increased protection when cold, (especially when starting a 'cold' engine) - when most wear occurs. The additive package is also often more extensive, using hydrocarbon molecular structures from other sources, (like refined gas and animal fats) to improve the base oil stock and probably higher-grade polymers than are used in an oil company's lesser products.
The general 'rule of thumb' that was quoted earlier of changing Mineral at 5,000 and Fully Synthetic at 10,000 rings true enough, (as true as general rules ever do) - at least in our experience...