Oil additives are for the birds.  Slick50, and the teflon treatments have no
beneficial effects on your motor and actually could damage it.  Anything that
pours like honey in 80 degree weather is just too thick to be putting into an
engine.  Most of it basically ends up sitting in the bottom of the oil pan.
The teflon stuff is a joke too.  Dupont (who invented Teflon) put out a news
release some time ago that stated that Teflon does not bond to metal until it
is heated to something like 1700 deg.
After talking to alot of different motor rebuilders and then hearing alot of
horror stories I would say your best bet for cold weather starting is multi-
weight oil and maybe some type of block heater (if you want that oil nice and
warn before you crank up the motor).
Thats my 2 cents.
Ken Wilford
Van-Again
John 3:16