Technically the molecules in the synthetic are not really smaller, in fact many of the molecules in petroleum oil are smaller.
Difference is that quality 100% synthetic all of the molecules are all the same size and are long chain that resemble something like a backbone.
Regular petroleum goes from super small to extra large molecules and every shape and size, it also has several naturally occurring contaminates like sulfur, wax, salt, etc that burn off and leave deposits and change the viscosity quickly.
Heat and Oxygen are what breaks oil down
Petroleum oil the minute you fire up your engine after an oil change has already changed and started burning off the very small molecules. This puts a larger load on the remaining oil molecules.
Not to mention what those really extra large molecules do is basically blanket the metal surface while the tiny molecules squirt around burning off. What this does is actually prevents the oil from doing its job which is to transfer heat from the metal to the oil. This is not a good thing considering oil is supposed to do about 50% of your engine cooling. Also having sizes from very tiny to extra large creates more friction which creates more heat which breaks down the oil faster.
Its sort of like if you take 2 plates of metal and put thousands of the exact same size ball bearings between them it should have very little friction and roll very easily, then take bearings the size of BB's up to the size of a baseball and many sizes in between and see how that rolls, lots of friction.
Petroleum oils have oxygen IN the base stock as a contaminate from the get go and because a large percentage of the molecules in petroleum are tiny they burn off and form a varnish, the sulfur burns off, the salts, the wax and on & on and all these things burning off and turning to acid breaks down the oil way quicker and cause sludge. What sludge is, is Oxidized oil and burned contaminates.
Real 100% synthetic suffers from none of these problems, no naturally occurring contaminates all the molecules are uniform in size (monomolecular) have a higher TBN to start with, don't have any wax so are usable in far lower temps and flash point is much higher. Which does not mean that 100% synthetic will never sludge, just means it is less likely and you really have to ask for it. Like never changing your PCV valve, using cheap oil filters and running the oil for 12,000+ miles regularly you would be asking for it.
First thing I would do is change the PCV valve which could have caused the problem in the first place.
Then get the cheapest petroleum oil you can get, its all pretty much the same manure, don't get high mileage or semi synthetic, just cheap 5W30 plain old oil.
I would run 3 bottles of AutoRX through it and change the cheap petroleum every 2000 miles for about 6000 miles.
Then start using a true 100% synthetic oil after that with just 3-4oz of the AutoRX in it with each oil change and change the oil every 6000-8000 miles or 1 year which ever comes first. I have ZERO personal knowledge of this Autorx, never used it, seems like it works but who knows could be snake oil.
OR
Use Seafoam treatment which many here seem to like and then use a decent but cheap synthetic like maybe the Rotella T6 5W40 and change the oil every 2000 miles for several changes. Using a good detergent oil and lots of changes could help to clean it out. Then start using a quality 100% synthetic with 6000-8000 mile OC
