I've switched to Synthetic in motors with more than 175k Miles and never had a problem. In fact, the motor ran smoother with less noise, like lifter tick.
Synthetic oil properties, like polar molecues, etc, does tend to flow more oil thru a leak. So, if the motor has lots of leaks, synthetic may make it worse. Give it a try, if it leaks to bad, then switch back to conventional oil, if your not willing to fix the leaks.
I'm not much for these schools of thought about crud holding an engine together. Think about what this is saying. The motor is being held together by dirt and garbage, touch it the wrong way and it will all fall apart. Oookaay, but you drive all over the place with the same motor?
This is like the school of thought about transmission problems, where you get someone that warns you not to change the fluid and filter in a tranny thats not working right, because it might do something to the shifting that will only make it worse.
OK, sure its possible that changing fluid or switching to a different type of fluid could cause dirt to break free or changes that pushes your motor/trans over the edge and it fails or malfunctions. BUT, the fluid is not the cause, its the motor/trans being in such bad shape its was on its last legs and was going to fail any minute is the true cause of such a thing. Its extremely rare, but if its happens, it was only brought on a few miles earlier than it would have happened if you done nothing at all.
So guys, if you think dirt might become unlodged and destroy your motor from switching oil types, then rebuild your motor because it needs it. If switching to synthetic is going dislodge enough crud to damage the motor, than there must be enough in the motor already that it could break free on its own at anytime and do the same damage. Fresh oil will clean better than old oil, so by this logic, if I have a dirty motor, NEVER CHANGE THE OIL AND IT WILL LAST LONGER because you never want to take a risk of breaking some of that dirt free and having it float around the motor.
All oils have cleaning properties and they also have the potential to create sludge and crud as a byproduct of their use. Using the proper oil and appropraite quality in the right conditions with a correctly working motor, the cleaning properties of the oil should outpace the byproduct production of sludge and crud.
If sludge and crud is building up in the motor, its because the byproduct production is outpacing the cleaning properties of the oil. It could be because of the poor quality of the oil, something is wrong with the motor (malfuntioning CCV system) or the conditions you use the motor in, or how often you change the oil.
Synthetic oil does NOT have some miraculous cleaning property or high detergeant additive package to clean up motors. Synthetic oil, since it so well engineered to do its job, contains very little of the byproducts that make sludge and crud, it resists breakdown much better to help fight it turning into sludge and crud, as well. This why motors running synthetic for a long time are so clean, they don't leave behind any deposits/sludge/crud like other oils. They may clean as well or a little better, but they aren't some sort of SUPER CLEANER, they just don't leave behind deposits like regular oil does.
If your motor is extremely dirty, switching to synthetic should clean it up, over time. If your motor is so dirty that switching to synthetic breaks free chunks of dirt that damages the motor or lets seals leak, then you motor was toast to begin with, the new oil only brought on the failure a few miles earlier than it would have happened on its own. Like I said before, fresh regular oil will clean better than old, as well, so wouldn't fresh oil have the potential to break off dirt chunks as well?
I've seen lots of speculation about what can go wrong when switching to synthetic oil in high mileage motors, but I see very little actual cases of people complaining of actually suffering any of that speculation. Instead you see a lot of endorsements with people saying that switching to synthetic was an improvement.
Weigh the pros and cons and decide on that, I just don't believe you should believe you should make decisions living in fear that your motor is being held together by dirt/sludge/crud and anything you do to improve it may make it fall apart. Its not going to happen, if it does (your that 1 in a million) then it was going to happen anyway, because the motor was ruined to begin with, you just didn't know it yet.