Clay,
You can use either/or. I use 10w-30 in the summer/warm climates (Southern
Cali.) and I use 5w-30 in the winter/cold climates (alaska). So it all
depends on where you live and what temp is pretty constant at that time. A
perfect example is Kansas. It gets bloody cold here in the winter so I need
the thin 5W when I first start my truck, so that all the parts get lubed
quick. But when it warms up to 90 like it did today for the first time in
the season, I will switch to 10W-30 so that the oil is a little thicker at
startup. The truth is that, 5W-30 flows at about the same consistency at
freezing, that 10W30 does at 80. You should look at your owners manual
further and it will tell you at what temps you should use what oil. It has a
whole chart in there for it. Also, if you have a chilton or haynes manual,
there should be a chart in there too. There is one in mine for my ranger.