No, you can even mix synthetic with conventional oil. It will do no harm at all.
synthetic to conventional - ok conventional to synthetic - no
Yes, you can even mix them.
Yes, you can even mix them.
yes
No it is fine to switch back to regular oil.
Yes you can. Contrary to a popular belief it does not hurt to switch between or mix synthetic and conventional oils.
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.
If you have kept regular oil changes, no prep is required. Just go and put the synthetic oil in.
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.
yes it is fine but of course synthetic is the best to use if you can. Double check your oil requirements with your owner's manual. If it states that normal oil is okay, then go ahead. If your owner's manual dictates Synthetics, then DO NOT switch.
Only if the engine has low miles or no sludge build up - synthetic oil captures the carbon within its molecules and if there is to much carbon buildup (sludge) it will release the carbon into the molecules of the synthetic oil and run it through the engine - once the filter is bypassing there is nothing to keep the "liquid sandpaper" from wiping out the bearings.
It is not a very god idea if that is what the manufacture requires, conventional will leave sludge and gunk in the engine.