I have two Sierra Designs 15 degree synthetic (Polarguard 3D) bags from about 20 years ago that have always been stored uncompressed and well cared for. They are as warm as they ever were and really comfortable. I wish someone still made one like them--mummy cut but generous, stretch built in, pad strap attachments and a seemingly very durable synthetic insulation. Somewhere along the line I got the idea that Polarguard is supposed to be very durable, but with some other trade-off in drape or warmth-weight efficiency, so you may want to investigate what type of synthetic fill the bag you are looking at has and how durable it is reputed to be.