Check the owners manual for starting instructions in cold weather
My 1977 300D sat for two days in NJ with temps at night in the single digits. It was around 10 degrees when I gave it a crank, and my battery is a little low on juice.
Owners manual states, "at ambient temps below +23 F, depress the accelerator at least half of its travel while starting".....also "at ambient temperatures below -4F, depress the accelerator three times prior to starting".
I used the -4F approach and it fired right up.
Althought I think a block heater is a good thing and synthetic oil helps, but if the valves are adjusted properly, glow plugs are operable, the beast should start.....but the method used does make a difference.