I third or fourth the motion on synthetic oil.

I was running 15W40 and when it got below 20F I started having trouble. When it did finally start at about 32F it was only after I had cranked on it several times and got the oil flowing finally.

You could tell that all the sudden it was spinning faster.

I changed the oil today to Rotella Synthetic 5W40 and it spins/starts like a champ.

It's 21F out right now and I had no problem.

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1993 W124 300D
-297K on the clock as I type this.