Dieser Text wurde mir von SAVVY zugeschickt.

Cold starts spell disaster for cylinders. A single unpreheated
cold start (particularly at temperatures below 20 F) can
inflict more cylinder damage than a thousand hours of
cruise flight! Contrary to popular belief, cold start damage
isn't caused by lack of lubrication,but rather by loss of piston
-to-cylinder clearance. This requires some explanation.
When an engine is cold, there is quite a lot of clearance
between the piston and the cylinder walls—usually more than
.010" of clearance. This is necessary because as the engine
heats up to operating temperature, the aluminum piston will
expand about twice as fast as the steel cylinder barrel will,
and the piston-to-cylinder clearance will get a good deal tighter.
And that's okay. But it's crucial that there always be at least a few
thousandths clearance between the piston and the cylinder
wall, so that the the oil film is not breached and metal-to-metal
contact is avoided.

During a cold start, the piston heats quite quickly, but the
cylinder warms up much more slowly because it has vastly
greater thermal mass and is covered with cooling fins and
bathed in frigid air. Consequently, there is often a period of
time—where the piston is up to temperature but the cylinder
hasn't caught up yet—when the piston-to-cylinder
clearance can actually go to zero and result in metal-to-metal
scuffing of the piston and cylinder walls. That's why cold starts
can be so devastating to cylinders.

Guidance to Savvy clients is to use multigrade oil (preferably
Phillips 20W-50, never Aeroshell 15W-50) during the winter *IF*
they anticipate the possibility of unpreheated cold starts in sub
-freezing temperatures. If they are sure that all starts in
subfreezing temperatures will be preheated (Tanis, etc.),then
we recommend using W100 all year around.