Synthetic oil verses mineral....
Hiney, I'll try to be simple and quick on this reply, since this is a topic that has many threads in the forum already.
When you get down to the basics, it's about the molecular structure of the synthetic verses the mineral. When oil was made as a lubricant, it was a process where it was determined that if you keep this "dinosaur juice" flowing over a bearing surface, it would remove (or minimize) the heat which is created by friction.
With the synthetic oil, what science and technology did was to breake down the molecular structure and found that they could make oily (very slippery) molecules that locked together and bonded to the bearing surface to create a more slippery surface, therefore, creating a lot less heat.
Imagine the way a honeycomb fits together. This would be how the molecular structure of the synthetic oil looks under a microscope sitting on a bearings surface. Now, take a shotgun with bird shot in it and give that honeycomb a blast from the shotgun. See all the holes in it. Well, these represent the molecules of the oil that does not bind or lock together, creating areas of the bearing in contact with the surface.it a blast.
Now, to ask why you can go one way but not the other.....that is because, the detergents or additives of the mineral oil are compatable with the synthetics but the synthetic oil composition is not compatable with the mineral. This will basically apply to "full synthetic" and not synthetic blend, since synthetic blend is formulated with a mineral base stock, by most manufacturers, you are able to go back and forth. But for me personally, in my opinion, once you upgrade, why drop back down to something known to be a lower standard (that's in anything you do in life).
I don't know how to make this any more simple than that. I use full synthetic in my trucks, my ski and my boat. I do use mineral in my lawn equipment. And there are other applications that you can't use synthetic. But, for the most part, the synthetic oils are by design, the best lubricant made to make the life of a bearing last longer and keep from making so many deposits on valves and heads..............
