Personally, I see nothing wrong with picking a number on the conservative side and going for it.s I do consider it guessing, but with modern oil guessing is usually plenty good. Better to do better than to do worse.
However, in my case, I view that as guessing. If you want to stop guessing, in each cycle (winter/summer/etc) have a lab test when you change it, and find out just how good that oil still was.
After years of this constant guessing, I had my Sienna tested with 8500 miles of non-snow zone highway driving, tested, and found that oil was still in good shape, could have gone well over 10,000 miles. So, until I had to park it in the USA, under the same conditions as tested, I changed it at around 7500 miles.
A different car, such as the Mexican 2009 Sienna I have now, I will tend to change it more often until and unless I get a chance to test its oil as well.
When you say extending the life of the oil that implies the oil is worn out or nearly so. IF you test it, you may find it is not worn out at all. Of course, you may also find out it is more worn out than you thought. Depending on your use and the car itself.
There is another reason to test it, and the condition of the oil is trivial by comparison. The test tells you great details about the condition of the motor. The bearings. The valves. Blow-by if any. Head gasket condition. If anything goes wrong in your motor the oil test will tell you, but only if you have your oil tested.
I use Blackstone, because it’s the only oil lab I have heard of. If it’s a good lab, which one you use probably doesn’t matter.