When the first time I started using Flexible OCI on my 2000 BMW 528i, it does have the maximum upper limit set to 18,000 miles! Of course it won't be that long the count-down 5-bar system will tell you changing the oil, at about 10,000 ~ 12,000 miles like you said. But I never waited that long to change oil, usually at 5,000 ~ 7,000 miles or 1 year with full synthetic oil and reset the Flexible counter.
For 2016 Mazda CX-5, the Flexible OCI has the maximum upper limits set at 10,000 miles or 12 months. It won't pass that before the oil change wrench indicator shows up. For 2017 CX-5 the upper limits on Flexible OCI is 7,500 miles or 12 months. From my experience on CX-5's Flexible system for 19K miles and 2+ years, I usually get 30% oil life left at about 5K ~ 6K miles with all city driving, and that's the time for me to change oil with Mazda moly.
Having split my adult life between the US and the UK, I think a lot of the "science" behind oil changes is based on historical norms and marketing.
I checked the UK Mazda service intervals and these are set for 12,500 miles which is quite common for vehicles in the UK. The Brits don't question it as the marketing of synthetic oils gives them the peace of mind that longer intervals are reasonable.
In the US, there is a more conservative approach to oil change frequencies. The oil companies here don't push the qualities of their engine oil as hard as in the UK so there seems to be a reluctance to go much beyond 5,0000 - 7,500m.
From what I've read, synthetic oil resists breaking down much better than it's Dino equivalent. It seems that it's almost unnecessary to use synthetic oil for 7,500m intervals as their real benefit is providing protection for double / triple that distance.
On the other hand, Americans demand tires that last 40, 50, 60,000 miles which is generally not the case in the UK. The Brits are marketed to buy higher performance tyres which wear out much faster.
Just my observations. YMMV.
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