This cost argument is really frustrating. Like Bill said, most people don't pay only $2 more for synth. In general synth is 2x more expensive. Like Bill said, the cost difference IS significant.
Moreover, if your really do have $10 or $20 bucks to spare, please spend it on a better higher quality food. Treat your OWN engine (heart) to a better diet BEFORE your car. Which one is more important??????
Now Bill, your UOA may not be typical. Consdier your driving pattern and what those 5.8k miles consist of. I believe you do a lot of highway driving, which of course is much easier on an engine. Many people on the other hand do more city plus use congested highways, so tend to get stuck in traffic more. This is especially true in winter with bad weather. So most people encounter a severe service during winter. Again, your roads might be better, less traffic and congestion, not sure.
For example, I drive 80% highway. Yet on bad days (snow/ice) my commute can double timewise. Thus my engine actually ends up working twice as long for the same distance covered, often with more acceleration and breaking. Sometimes I get stuck behind a salt truck, so back to 2nd gear. And so on.
The point is that engine generally works harder during winter due to inclement weather. There is more moisture, salt. It is probably safer to go with synthetic or a blend and avoid having to change my oil at freezing temps in the middle of the winter.
While you UOAs are great, I am not sure that they are the proof of Castrol being able to handle winter driving for others.
Of course, it depends on how cold it gets, how much snow, rain, etc. Some people, maybe most drivers in US would do fine with dino like Castrol. Still, northern states like NY, Maine, Vermont, etc. as well as Canada probably could use more synthetic oil.
Again, do use synthetic FOR A REASON, and not just because it is a couple of bucks more!!!!