Yes, and you can even mix them together.
That's okay.
yes you can mix oil brands. it is not recommended to do though. As long as the weight of the oil is the same.
Yes, you can switch brands with no problems at all. Just make sure you use the exact weight that the manufacture of your vehicle recommends.
Nothing if they were approved oils. You can mix 10w30 with 5w20 etc. Not a good idea but you can do it. You can mix brands with no problem.
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.
Yes, you can mix conventional oil with synthetic oil with no problem.
Yes
It is a gas/oil mix motor, you don't change the oil.
Nothing happens, they just mix. You can buy "blended" motor oil.
Motor oil is insoluble in water and it is denser in water. Therefore, motor oil will not mix with water. It will remain separate.
This may not be so harmful if it was only a gulp. The two oils will mix okay and may not do any damage.