[at-l] Washing synthetic bag?

[MrCharlesENelson@xxxxxxx (MrCharlesENelson@xxxxxxx)]

'Ello mates!  I have a North Face Cat's Meow that is about 4 years old with
many a night's use on it.  It seems to have lost much of it's original loft
and warmth- I get cold in it now in the mid 30's - way above it's 20 degree
rating.  I've heard that over time the insulation (polargaurd 3D) collects
oils from perspiration and loses loft-and hence the ability to trap warm air.
 I've never washed it-always heard that you shouldn't wash a bag unless
absolutely necessary.  That true?  Will washing it restore some of it's loft?