OP: yes you can mix oils. That's how they get "blends"...synth + conventional. As far as high mileage or not, do you have any leaks right now? HM oils have seal swelling agents in their add packs that help reduce leaks. My '02 Accord has 133k and no leaks so I use conv oil. No problems.

I've mixed oils occasionally, don't make a constant habit of it, but never had problems. Castrol is probably just changing their labeling or marketing etc. I've mixed synthetic with petroleum-base oil too.

I used regular (not high-milage) oil for well over 100k miles on several different cars. For 220k miles on an 89 Saab. I wouldn't use "hi-mile" oil unless there's a specific issue you're dealing with like leaking seals.

No leaks that I am aware of. 93K now.

Is Castrol GTX the best oil?

I've used the cheapest oil available every time I change the oil. Our wagon now has 286k miles and still doesn't burn oil, leaks a little. Use whatever oil of the right weight you can find, check it often and change it when its called for or a little sooner (3k-5k miles) and your Accord will last a long time.

I would say that whether or not "brand A" is better than "brand B" is subjective and everybody has their own personal preference. As long as you are using the correct viscosity and changing it at the proper interval, Castrol should suit you fine.

I've been using Castrol GTX in the non-turbo cars (various Hondas) for many years.

My kids' 2007 Civics have the oil-life monitor, and according to oil-analysis it's been OK when changed at 10% per the monitor. That worked out to about 7,000 miles for their driving habits. We had a 2003 Accord (4-cyl) and I was always afraid to go 10,000 miles on the oil, but that's what the owner's manual said. I usually went about 7k.

Old wives tail. You can mix, you can go back and forth, whatever. Think about it this way....there is conventional, there is synthetic, and there is "synthetic blend" which is both. It speaks for itself. If you couldn't mix the two you would never be able to put sythetic in a car that came factory with conventional and vice versa because you don't get every drip of oil out when you change it. I have been using Amsoil for years, and all their fluids are completely 100% compatible with any conventional product.