2015 Silverado LTZ

[Dad's72]

Hey y'all. I recently got me a 2015 Chevy Silverado LTZ Texas Edition. I have final put a put a few miles on it and it is time to change the oil. My question to any of y'all who have or do own this model of truck is this. I would imagine that the previous owner and dealership probably used conventional oil on this truck would I do it harm by switching to synthetic at this point in the game? It has exactly 40K miles on it as I type. I was reading that by switching to synthetic oil and draining the differential and putting synthetic in there I could gain mpg. I get pretty decent right now. I average 19.5 in the city and when I took it out of town on the highway I got 22 mpg on that tank.
Also one other question. I see that they put in a 200 degree thermostat from the factory. Would it hurt to go to a 180?