I don't know of any name brand synthetic or partial synthetic which will attack any current gasket or seal material.
If you have existing bad gaskets or seals, and the holes are covered by sludge or varnish, synthetic oil (or another brand of non-synthetic) can clean out the gunk and you will find the leaks.

I have changed cars to AMSOIL with over 200,000 on them-- including my own Dodge van, purchased from a motel with over 200K on it. Before I bought it, it was burning oil and smoking on startup. I flushed it, changed it, and found the oil consumption to be a quart in 600-700 miles. Bad, but liveable, since I only used it to haul products. A couple of years later, consumption leaped to a quart in 150 miles. Had the engine checked out, and found two bad cylinders in block (not just valve seals/guides), so dropped in a remanufactured engine.
Of all the cars which I know of, only 3 or 4 had oil leakage problems develop.
These were either cars which had known problems or had been poorly maintained.

A choice you have to make for yourself. If you do change over, be sure to use a good engine flush first. If you don't, or won't, flush, I suggest using 1 quart of synthetic with the remaining quarts of non-synthetic and run that for about 1,000 miles to help clean out the gunk.