As long as it is not a requirement of the engine, such as with VW's, Corvettes, Vipers, Mercedes, and some other higher end cars, then there will be no problem. So if you have an Accord, like the category you put it in, then you should be fine.
use it, it is ok to mix
yes, if the car manufacturer dictates that regular oil is acceptable. If your car requires synthetic oil as dictated by your owner's manual, then DO NOT use regular oil. May void warranties if your car is still covered.
Yes it is ok, but, I think that you will be wasting your $$$$, as you will still have to change the oil as per the same time as regular oil.
synthetic to conventional - ok conventional to synthetic - no
As long as the oil had the right rating it'll be OK.
It's okay to use Synthetic oil on any vehicle but bare in mind, if your vehicle has previously run on regular oil you will need to drain all the oil out completely in order to switch to synthetic, once you run on synthetic oil though you have to keep the oil synthetic because switching between different types of oil will really mess up your vehicle.
You should use SYNTHETIC not REGULAR which I've learned from my own personal experience. Regular oil in a 2002 camaro only kills the oil pump. It's a rather expensive lesson to learn also. Don't let anyone tell you that putting regular or synthetic blend in your 2002 camaro is ok it's not.
Yes, you can use synthetic, conventional, or a mix of the two.
Yes it is ok.
Yes
You need to use the type and weight oil that the manufacture requires. From 2001 semi synthetic oil is recommended for the C230 Up to 2001 Mineral oil is OK for use in the C230.
If the enigine calls for 5w-30 then the only real reason that you switch to a thicker viscosity is to lessen oil leaks? If yes, than there is nothing wrong with that.