I have various tights, various long sleeve tops, all synthetic except for smartwool socks. For 40 degree (plus or minus), I use two layers on my legs, three+ on my torso, a head sock on my head, face and neck. I also use a heavier jacket with hood, or sometimes a wind/rain shell, and long finger insulated biking gloves. Granted, I'm 68 and don't like being cold. Also, I ride with a full frame bag and a handlebar bag, so I have room for the layers I take off as I get warmer, putting them back on near the end of the ride as the sun goes down. All of my gear, including my underwear, is "gravel specific".