The reason why you can't go back to regular oil after synthetic is because it has the potential to cause issues with the engine seals and engine components and may lead to the engine start to leak oil more.
The engine though will not be damaged if you do go back to regular oil after synthetic as it's generally safe for most engines to switch back to regular oil after synthetic.
If you switch from synthetic oil to conventional oil, any gaskets and seals which had no cracks with the other larger molecules, could begin to deteriorate with expansion and heat when you use synthetic oil.
There is a downside to synthetic oil which includes a potential for reduced fuel economy or mpg at highway speeds, higher cost and even the possibility of revealing existing oil leaks in older engines.
Synthetic oil is not always needed in some engines and could do more harm than good.
Although synthetic oil does have some good benefits but also drawbacks.
Synthetic oil can also cause drying and swelling of the engine seals and gaskets which could also lead to engine oil leaks.
The reason why you shouldn't use full synthetic oil is because the use of full synthetic oil can sometimes cause leaks or even increased oil consumption.
Full synthetic oil is not recommended for high mileage engines which have always used conventional or regular oil, or for rebuilt engines and engines without wet clutches.
Synthetic oil is also more expensive than regular or conventional oil and it's not always necessary for vehicles that do not experience extreme driving conditions or frequent hard driving.
Conventional engine oil is fine for most vehicles.
When you mix regular oil with full synthetic oil, the regular oil will dilute the benefits of the synthetic oil and can reduce the full synthetic oils performance and protection.
However when you do mix the regular oil with full synthetic oil, mixing the two oils will not harm your engine.
Your engine will still be lubricated properly but the benefits of the full synthetic oil are reduced when you mix both regular oil with full synthetic oil.
I've had to mix regular oil with full synthetic oil several times when I ran out of full synthetic oil and needed to add oil to my engine.
Nothing bad happened to the engine and the engine is still running fine.
It's best to avoid mixing the regular oil with full synthetic if you can, but it won't do any harm to the engine if you do mix the oils together.
The only thing that happens if you mix the two types of oils is that the regular oil or conventional oil keeps the synthetic oil from performing to it's full potential.
Also it's best to not go back to regular oil after you start using full synthetic oil although you can if you need too.
Synthetic oil has smaller molecules and higher densities.
So if you switch from synthetic oil to conventional oil, any gaskets and seals which had no cracks with the other larger molecules, could begin to deteriorate with expansion and heat when you use synthetic oil.