As a general rule, synthetic oil will be a better choice for your engine under a cold start situation and in extreme heat or cold temps. You can also safely increase your oil change interval (with a premium filter) to help with the increased cost of synthetic oil.

The viscoity will remain more consistant, the additive pack is generally better and synthetic has an edge in film strength (stays on your engine parts for a longer time) to help lubricate an engine that sits for extended periods of time.

 

I use 0w-20 in the winter and 5w-20 in the summer. Just took a trip from Indy to Hampton, VA last weekend on 5w-20 Quaker State Synthetic and averaged 38.5 MPG running up to 75 mph. I got 40.5 mpg going over and 36.5 coming home. I didn't think that was too bad for going through the mountains and some small construction areas. I always use synthetic oil in my vehicles and have for years, brands vary, but always synthetic.