Synthetic insulation permanently loses some loft in the first tight stuffing. Some ultralight bag companies ship their bags
unstuffed from the factory so you can avoid this.

What really kills synthetics is high heat. They really shrink
up if dried too hot. Tumble on air or hang dry to keep loft.

Down bags will last forever. My wife still uses a REI down bag
from the 50's. It was rated to zero then and still is good
for that temperature despite many washings and years of use
during her outward bound career. Synthetics on the other hand
tend to need replacing each year if used that hard. Wiggy's
included.

That said, for a car bag I would use a synthetic bag for it's
quick drying ability and for the low expense. I would store it in a duffle larger than the stuffsack that comes with it.