You probably have several things going here:

Too thick oil -- use 10W30 or thinner synthetic for winter use

Late injection and valve timing (worn chain, most likely -- check stretch and replace if necessary)

Slow starter. If you don't get 100 rpm or more cold, replace with NEW or BOSCH REBUILD -- the armature is bad, new bearings and brushes won't fix it.

Tight valves -- do a valve adjust. If tight, leave a bit loose and re-check in 100 miles.

In decent shape, an MB diesel should start with no problems down to about zero F without a block heater.

Peter

__________________
1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!