do not start the bike during winter for this reason...
Don't do that unless you are going to be able to ride it long enough to get up to full operating temperature. By just starting it and letting it run for a few minutes, you get condensation and other contaminants in the oil. The bike needs to be ridden so it is good and hot to burn these off.
It really is better to put it away properly and leave it until you are ready to ride again.
gasman's recommended winterization checklist...
Here's what I, myself, do to my bikes before I put them away.
Add stabalizer to gas... ride 10 miles... then home
change oil and filter.
bring motor up to temp
wash n wax bike throughly
top off gasoline to brim and if it has a petcock turn it to off
check/adjust air pressure in tires
Put in corner on stands. Any tire still touching ground should be on wood.
Pull battery and put on battery tender.
In the spring...
put battery back in bike
get out of corner and uncovered and off stands.
start bike and bring to temp
check for any leaks oil or water
turn off bike
check air pressure in tires
ride bike thru 2 tanks of gas.
change oil and filter again.
replace spark plugs!
Ride it like you stole it!