Christina,

I'm sure you'll get a lot of excellent suggestions here, but I'll take a crack at it.

The effectiveness of insulation of the type in sleeping bags comes almost entirely from it's thickness. Synthetic fills are capable of being just as effective as down, and generally are just as effective as down for the same thickness... just not as light, and thus not as effective per ounce.

For carrying in a car, extreme light weight is not as important as for carrying cross-country on foot, and I'm sure there are some excellent deals to be had if you can go, say, a pound heavier. For your use I wouldn't hesitate to consider synthetics. THat's NOT saying that the particular bag you're considering is capable of -20, that's another question, and usually not a simple one.

One consideration, though, is that (unless synthetics have changed quite a bit in recent years) they tend to lose loft when kept packed tightly for long periods. Something to think about, anyway. To some degree, it may depend on the size of your fiance's vehicle.