I'm no expert but I do know 2 things about symthetic oils of note. The general consinsious is it doesn't burn as readily. That for instance if you put symthetic in an engine that burns oil it will "burn" less of it, but likely use more cause it's a smaller molicule and leaks past seals more. That said not all symthetics are actually synthetics. Back in the day it was decided after some court battles that highly refined dino oil can be sold as synthetic. So most lower cost "synthetic oils are what's called class 3 synthetics which are highly refined dino oils with synthetic additives. Finding out which is which can be tricky, not something the manufactures make easy to find out. Amsoil and redline I know are true syntheitcs, I think castrol and mobil 1, and pretty sure royal purple, pennzoil, etc aren't.
Just my $.02