I do not want to point fingers. I do not want this to turn into a flame war. So if you feel strongly enough on the subject to react in anger please step back from the keyboard and take a moment. I am not naming the vendor that made this scope. I am not naming the vendor who made the other scopes I have seen this problem with. It doesn't matter who made it. What matters is this can happen with oil spaced triplets. Once is a fluke, twice is a coincidence, three or more times and something is going on...
... This will be the third one of these I have personally seen develop this problem. So far it has been due to storage for long periods (1+ year) in a horizontal position. I am guessing the two ways to prevent this are:
1. To use the scope regularly and often
2. rotate the position that it is stored or perhaps store it vertically(?)
In each case the vendor has made things right for the owner. Unfortunately there was still a cost involved. And for larger apertures the shipping price can become prohibitive by itself regardless of the cost for the vendor to perform the repairs. And in all three cases these were not 20+ year old scopes.