I'll speak only to what can occur AFTER changing to synthetic from dino.
Here's what happens to your seals on an older motor with a pure history of dino, and why they stay sealed unless you change to synthetic:
If you've ever replaced aseal you'll notice that hey have two "lips", inner and outter, with a groove in between. When they're new, soft and pliable a certain amount of oil creeps past the first lip into the oil receptacle (the groove) and gets trapped between the two lips, providing lubrication to the rotating countershaft (kicker shaft) or whatever protrusion the seal is trying to seal, and keeping the spinning shaft from tearing up the seal.
On a motor that's been run for a long time on dino without frequent changes the sludge from the dino begins to seal the motor from the inside. Less (or no) oil reaches the receptacle and the outter lip dries out and begins to shrink around the shaft. Its I.D. is then increased due to lathe effect from rubbing the dry shaft. Your seals are essentially junk, but they still seal, partly BECAUSE they're junk. Sorta.
Then you switch to synthetic. It removes all the sludge sealing the back (crankcase) side of your seals. To a degree The newfound oil flow restores the seals and the outside lip which has increased in I.D. "plumps" back up and the i.d. becomes even bigger and leaks like a sieve.
The same thing happens to your base gasket (and to a lesser extent your valve cover, side cover or any other o-ring type gasket), except that it's simply a flat plate of fibrous material. A fine buildup of sludge may or may not be all that is keeping it from leaking from the inside, out.