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No synthetic oil on the market today will attack any gasket or seal material.
Period!!!
If you have a false seal caused by a buildup of sludge or varnish, and the true seal has dried, changing to synthetic (or even another brand of oil) can cause the gunk to be cleaned away. Then you'll find the leak that was already there.
My 1985 Saab 900Turbo has a leaking valve cover gasket. Gee, must have been the synthetic oil which was put in originally about 180,000 miles ago (no, not the same batch of oil).
Your assumption that it was time is correct.
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