Your car should start in 10 degree weather if everything is up to spec. Is your compression poor since you have 369,000 miles on the car? How about the battery-I used a large Interstate battery in my 1983 300D that really turned the engine over. Have you had the valves adjusted recently to maximize compression for starting? How about the glow plugs-are they functioning properly or in need of replacement? Synthetic oil would also be worth a try but many have indicated that it can cause leaks to appear on higher mileage cars. Have you tried waiting longer after the glow plug light goes off before trying to start the car? It is my understanding that the glow plugs will continue to heat for a certain time after the light goes out.

Bob I.
1983 300D (May 1983 to May 2000)