I would probably stick with 5W-30 instead of 15W-40 and here is my thoughts on why my thinking is this way. I have used my Honda Snowblower in temps from 5 degrees up to 30 degrees doing my 100 foot driveway as well as my neighbors, sometimes from 1 to 2 hours. I have an infrared gun, I know this is not totally scientific, but the highest oil temp I have recorded is about 175 degrees fahrenheit.
I also notice with my riding mower that in 90 degree plus weather I record an oil temp with 10W-30 of about 205 degrees, the other day mowing it was 40 degrees out and I only recorded an oil temperature of 185 degrees. My thoughts are with air cooled equipment is that the colder the temperature the lower the oil temperature will be.