Synthetics are also available for older thicker oil engines. There
is no such thing as engines designed not to use it, as it will do
all engines.

Actually, you can get problems with air cooled engines that are not designed
for it. Mobil (I think) formulated a fully synthetic oil for aviation
reciprocating engines. Unfortunately, not long after release, they started
having lots of reported failures. The majority of light aircraft engines are
air cooled, which usually means the oil gets to a higher temperature and is
required to work somewhat harder. Coupled with this, the engines run on a
highly leaded fuel, which the synthetic oil had trouble with, as it was
unable to clean the lead away. I have no idea if the same issues would
affect air cooled road engines, like older VWs and Porsches, especially if
they run on unleaded. However, there most definitely are some engines that
can't use synthetics safely.

OTOH, I agree that for a water cooled engine, anything other than a fully
synthetic oil is complete folly.

Christian.