Fuel starvation when very cold - w123 300d

[Zacharias]

My 1985 300d starts enthusiastically (after a few turns over) down to about -15 C (around 5 F). The story changes at -20 C (zero F).

Then it tries to fire, doesn't, and seems to then starve for fuel and requires about 10 seconds cranking to finally start to fire. Battery is charged and the starter is turning fast.

When it does fire, there is no evidence of unburned fuel in the exhaust. It's clean.

I initially thought it might be gelling fuel... I already filled up and added Diesel-911. Maybe a very slight improvement.

Rotellla synthetic 5w40 in the crankcase. I changed both fuel filters when I put in the oil, late November. Prefilter looks fine.

It's slightly worse when the car is parked facing into my driveway (very slight uphill slant).

Ideas? I've had cold weather issues before (on other cars), but never one that only showed up when moving to extreme cold from very cold.