I use them both. Up here, where it can get a little cold I use 5-40 synthetic in the winter. In the summer I use 15-40 non synthetic. The thinner oil makes a big difference when your on the minus side of zero.

I run Rotella 15w40 in the summer, and Rotella 5w40 in the winter. I ran 15w40 once in the winter and it took noticeably longer to build oil pressure

I run schaeffers 5w-40 synthetic oil year around

Typically 5w40 is synthetic and 15w40 is conventional...either one will work. Like Scott, I run Schaeffer's 5w40 year round...I got 24k on this fill, got to change it tonight!!