I recently drained some VX Semi Synth from the Westfield. Can't remember exactly how many miles it had been in there. Certainly not more than 4k or so.
Now, the car does get a bit of a spanking at times, but also had done probably 2k of motorway cruising on that oil, and it has a generous oil-to-water heat exchanger so the oil temperature doesn't exceed 100 degrees unless it's being given real abuse.
What struck me was how thin it was (in fact, what prompted me to change it was a drop in oil pressure).
I compared the flow rate through a funnel with the oil that was about to go in (a 5w30, so less viscous all round in theory) and the Semi Synth was a good bit thinner.

In other words, clarity of the oil isn't the only factor. If the oil is staying clean and you're tempted to leave it for longer intervals, I would use a better oil than the VX 10w40, personally. Which will cost more, of course, so perhaps it's better just to change regularly.
Kevin