The leak issue is that synthetic oil & conventional have different "swell rates" ie the seals on your engine are affected & swell differently is you change from typically conventional to synthetic oil, particularly older engines.
Consumer Reports took a fleet of taxis, had the engines all rebuilt, ran them 100K mi, tore them down & carefully measured the wear, comparing all major brands of oil & conventional vs synthetic, the results were no difference in engine wear, also the mpg was unaffected in the test engines.
I used to change conventional oil like clockwork @ 3K intervals, then I went to synthetic for a few years, now I'm back to conventional oil & 5K intervals minimum, the conventional oil is that good nowdays, and the engines are a lot better.