First, understand that there is no generally accepted technical or legal definition of "synthetic".

Typical C-category 15W-40s are not considered "synthetic" but they do require more Group 2 and Group 3 base stocks to meet current oxidation resistance tests. Maybe you can call them "semi-synthetics".

C-category 5W-40s are generally considered "synthetic" because most of the base stock is Groups 2, 3, and 4, which is what it takes to achieve their wide viscosity range.

As a general rule synthetics have greater service life because of their greater resistance to oxidation, but most models where synthetic is recommended have oil quality monitors.

Pick a winter grade viscosity that's okay for cold starts down to the typical low temperatures you expect while the oil is in service, and change it at a frequency that makes reasonable sense based on the manufacturer's recommended service interval, how quickly you accumulate mileage, and whether the oil is conventional or so called synthetic.

Guys seem to get tied up in knots over what kind of oil to use, but it shouldn't be an ordeal if you have good information, but the trouble is that the world (and internet) are full of myths and misinformation about engine oil.

SAE viscosity grades cover a fairly wide range of absolute viscosities. If you want to fret over this look up manufacturer's specs sheets, which usually list pour points and absolute viscosities at both very low and high temperatures. For a given viscosity it doesn't matter if the oil is conventional or "synthetic" they will both flow the same at a specific viscosity.

Duke