What's the downside of synth. oil?

I've used regular motor oil all my life (mostly Phillips Trop-Artic or Valvoline) with no problems. I live in the midwest and it gets well below zero at times, as well as
very hot in summer. I'm mostly attratcted to synthetic (Mobil 1, etc.) because of the low temperature flow characteristics for cold winter starts. Does it afford superior protection at high temps as well?
If so, then why doesn't everyone use it??
I know it's more expensive but that should be negated by the fact it can be used much longer before needing changed. Are there any issues with switching to it after an engine has 130K? I just bought a '94 Caravan 3.3 which has been serviced at the dealership since new with conventional oil.
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I switched to synthetic in my 1997 Caravan 3.3 a couple of years back. The first fill of synthetic become black as ink almost immediately. So, I changed it after only a couple of weeks with a fresh fill of synthetic (changed the filter each time, btw). The second fill became dirty quickly as well, but not nearly as quickly. Now I'm up to 6 months or 5K miles between changes. The oil responds fairly normally as far as the rate that it becomes dirty. No problems, no leaks. The vehicle runs great...always has.
I've been told that switching to synthetic after many miles using dino will dissolve some of the accumulation and build up of gunk/sludge. Based on how quickly the first few fills of synthetic became dirty, I think I believe it.
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same can be said for high detergent oil

The
will
how
it.
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