I switch back and forth every spring and fall. 5W/30 for our cold winters and 10W 30 for the hot summers, with no problems.
Most owners manuals give some kind of temperature range and oil viscosity recommendations. For example, my '97 Cadillac says 10W 30 on the oil filler cap, but the owners manual suggests 5W 30 of the ambient air temp is consistently below 60 deg F. Above this temp, 10W 30 is recommended.