When I switched to synthetic (5w40 rotella) in my cars, I was shocked. It seems like I bought 10 degrees of colder starts. All four of ours have started after sitting out at school for 7-10 hours.
And its been pretty cold this year.

Something else interesting that I discovered was that if I just changed a filter at 3k miles, I could extend my oil 6k instead of 3k. This is a point of debate among oil/diesel experts, but enough of them suggested it, so I did it. Therefore, the $12.83 per gallon (walmart costs earlier in the year) was pretty much the same as dino because I got twice the mileage. So far, this has been a nice strategy during the winter months.

Don

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DAILY DRIVERS:
'84 300DT 298k (Aubrey's)
'99.5 Jetta TDI IV 251k (Julie's)
'97 Jetta TDI 127k (Amber's)
'97 Jetta TDI 186k (Matt's)
'96 Passat TDI 237k (Don's
'84 300D 211k Mint (Arne- Undergoing Greasecar Conversion)

SOLD:
'82 240D 229k (Matt's - Converted-300DT w/ 4 speed