Any reason to not use Synthetic Oils?

No downsides (except price), especially if your car is newer than 1980 and you are using a synthetic oil made in the last 20 years. There were reports, when the synthetics hit the market in the mid- ‘70’s, that they were more prone to leak due to incompatibility with some engine seal materials. I wouldn’t consider these reports as fact, because engines back then tended to leak anyway and the Synthetic oil formulations have changed significantly since then. Reports that the piston rings on a new engine won’t break-in properly with synthetic are simply incorrect.

Synthetic oil cost is somewhat offset by the fact that they really do last longer in a typical engine. The reason is primarily
greatly improved oxidation resistance; this means the oil maintains its viscosity much better over time at the high temperatures in an engine. It is also the reason why a synthetic oil darkens more slowly in-service. Most engine oil darkening is due to oxidation (think of oil in a hot frying pan) not soot.