Can't speak to the synthetic oil issue, but it would seem that the garage would help quite a bit in the morning after a -15 to -20 nite. However if you don't get it back in the garage before you shut it off in that kind of weather, and it sits all day (nite) you will likely not see any garage advantage at that point.

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1961 190Db retired
1968 220D/8 325,000
1983 300D 164,150