Yes you can mix different brands but I recommend you do not mix different weights. You can even mix synthetic oil with conventional oil. But as the weight of the oil is critical, do not ever mix one weight of oil with another, no matter if it is synthetic or conventional oil.
That's okay.
No, you can even mix synthetic with conventional oil. It will do no harm at all.
Yes, and you can even mix them together.
Yes, you can mix conventional oil with synthetic oil with no problem.
Nothing happens, they just mix. You can buy "blended" motor oil.
Yes, you can not only switch you can mix synthetic with conventional oil.
You can mix part synthetic oil with full synthetic oil.
All major brands of motor oil are manufactured to the same standards. To pick a best if impossible. But saying that Synthetic is superior to conventional oil.
Yes, or you can mix them. Just bear in mind that if you mix them, you must maintain as if it were straight conventional oil and that by mixing, you lose pretty much all of the benefits that synthetic oil has.
Yes, just make sure the weights are the same so it will be compatible with your vehicle.
No. It will ruin an engine. The above answer is 100% wrong. You can mix convention oil with synthetic and it will do no harm at all. When you see them selling Synthetic blend that is what you are buying. A blend of synthetic and conventional oil.