No question about it synthetic oil is far slippier than regular oil. I have seen numerous engines run cooler with a switch to sythetic. This is from a reduction in friction.

Two drawbacks to sythetic. The molecules are smaller than regular oil so it will find leaks that regular oil does not.

Oil temps remain hotter with sythetic. Sythetic oil does not transfer heat as well as regular oil. Just like cast iron holds heat longer than aluminum.

No matter what kind of oil you use it has to be changed at short intervals. The darkness you see in your oil is the carbon deposits that get past the rings. Hint. Carbon makes good sandpaper.

I like to use half and half. But mix it yourself. The premixed stuff in bottles is likely very low in synthetic content.