I did read a reasonable rec. somewhere around here once.
Using the OM as a guidline and decreasing the mileage between changes by 20%.
Honda's 7500mi rec now become 6000. Using this method would at least give you the feeling that you are taking very good care of your car.

 

"Slightly used" oil is somewhat better than new oil. The oil film on the bearings takes time to build-up and settle-in. When new synthetic oil is introduced into the engine, it has a tendency to clean-off this film and gradually rebuild another. When this happens too often, more wear will occur. This is one reason Amsoil focuses on extended drain on their line of synthetics and their UOA (used oil analysis) regularly see decent to positive results.

Your 3500 kilometre OCI on expensive Motul 8100 (100% synthetic) is considered "slightly used." You're not using the oil to its fullest potential.