When a vehicle is older, its seals may have little cracks and leaks in it. Dino oil can cake and sludge up in the cracks, thus creating a seal. Also, some cracks are so small that dino oil isn't able to penetrate it. Synthetic's superior abilities may do the following: 1) clean out the cracks, thus allowing the oil to seep past 2) it lubricates/penetrates better so those cracks that were too small for dino oil to get past aren't too small for synthetic.

So a more accurate statement is that "Synthetics find leaks." If you have a pre-existing leak, get if fixed before switching. It will leak more after you switch if you don't fix it.
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Kenric Tam
1990 Volvo 740 base sedan (B230F)
My Volvo 'Project'