synthetic oil

I use what the engines need for use it sees.
You should too.
#1. Correct viscosity for the operating temperatures and clearances. Based on FSM or builder's recommendations.
#2. A multi-grade that will get the to target viscosity under the uses the engine sees.
#2 a. An engine in cold climate, that does short trips is where a synthetic or semi-synthetic is usually better.
#2 b. For cold winter use, compare "Pour Points". Synthetics are almost always lower for the same grade as Dino.
#3 Look at other stuff based on use.
#3 a. Non-cat flat tappet I still prefer zddp based anti-wear package, and some moly if possible.
#3 b. Cat equipped flat tappet, same but I look for somewhat lower concentration when running stock valve train. 800-1000 ppm.
#3 c. First few thousand miles after break in, conventional oil if at all possible.