15 000km to change engine oil should not be a problem, in fact, I realized recently that I experience better fuel efficiency after 10k, consistently hitting 14 to 15km per litre... I also use Lube control which also mentioned to change engine oil only when use Lube control 20 at 16,000km.
There are a few things that you might consider before stretching it :
1. The 15000km you mention might be a recommendation for overseas where people drive hours on the highway to work.
In Singapore, we drive in start-stop traffic and trips take a shorter duration.
Our driving condition is actually more severe on the engine oil and make it to degrade faster.
2. There are metal particles that go into engine oil from the metallic moving parts and get removed through oil filter.
If you stretch the change frequency, the filter may get choked.
Some people buy good oil that last 20k, but they still change oil filter every 10k because of this reason.
3. Engine oil experience high stresses and go through shearing, which lowers their viscosity over time.
If you consistently get good mileage after 10k, it could mean your engine oil has degraded and become less viscous - thinner oil gives better fuel efficiency.
Since you don't know how much the oil has degraded (unless u send for used-oil-analysis), I'm not sure if it's a good idea to use degraded oil for better FC.
Just my 2 cents.
