You need a starter that can spin the engine nicely, not just spin it. I replaced mine a month or so ago, when I couldn't start it with weather in the twenties and had to get jumped a couple times. I formerly had to block heat my car in any weather below freezing.

This morning the weather hit 12, and I glowed it about 30 seconds and it caught and ran smoothly. No block heater.

Do everything else first: valve adjustment, glow plugs, filters--diesels need clean air and clean fuel to start well.

Test your battery under load, and if you aren't sure how old your battery is replace it. Some say that batteries that test okay, if older than five years, should be replaced.

I run synthetic as well, 0-40 in the winter, 15-50 in the summer.

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Peter
1985 300TD 4-speed 212K
1992 400E 343K
2001 E320 72K