Why is it aircraft engines specify a single weight oil, i.e., SAE 30, not
mult-vis oils?  This is mysterious to me because aircraft engines must be
started under varying temperatures just the same as cars.

Once started and warmed up, automobile engines operate at a more constant
temperature than do aircraft engines.  If operating temperature would be
the criterium, then car engines would be better served than airplane
engines, yet we see mult-vis the rule for autos, but single weight for
airplanes.

Could it be that the multi-vis additives somehow detract from the
lubricating qualities which are better in single weight oils?  On the
contrary, could it be that most aircraft engines predate multi-vis oil and
were certified for the then available oils which were single weight?  There
are multi-vis aircraft oils, so perhaps later engines were certified with
multi-vis oils and the argument of superior qualities does not apply.

Also, I wonder why aircraft multi-vis oils are labeled for aircraft but not
cars and vice versa.  Isn't SAE 5W-30 the same whether it is poured into an
aircraft engine or a car engine?  Why is aircraft oil specially labeled and
costs more than car engine oil?