Go with Lycoming's service bulletin: if it's 20ยบ or less, leave it
along, unless you have a means to warm it properly. Our flying club
('69 C-172) has adopted this as our policy, but colder than that, the
FBO has heat at $20 / 15' {at the pilot's cost} (I think it would
take at least 1/2 hour do do anything good for the engine.).jbb
