What oil are you using? The starter sees more load when it's cold because the oil is going to be more viscous in the winter. My personal opinion (I'm in north-central Illinois now, but moving to Montana shortly) is to use a full or partial synthetic oil, something like 0-40W or 5-40W. If there is a 0 or 5-30W, that would make cranking easier. On the battery issue, you can have a 700CCA battery that is smaller or a 700CCA battery that is larger. The "larger" battery is going to give you a longer cranking period before it begins to weaken. I can't find (maybe it's still made, but I haven't found one) the full length battery for my 69 220d, so I'm going to install a second battery and run it in parallel with the "main" battery. The 220d really needs to spin for relatively fast starts, which isn't all that easy when the glow plugs are drawing 50 amp's. I'm not suggesting you should put in a second battery, but if you're having trouble starting in the Wisconsin winter, probably should first look at the soil you're using; secondly, is you're starter weak; thirdly, does the replacement battery physically fill the battery tray (if not, then I'd go with a battery that does physically fill the battery tray), as the total area of the battery plates is proportional in inversely proportional to how quickly the battery weakens during a hard start. I'm sure other's will advise you, which is good, as perhaps I'm wrong, but this is my considered opinion. Also make sure the battery terminals and cables are clean and you have a good ground at both the engine ground and the starter ground (which goes to the chassis, at least on my engine).
I forgot to say that the temperature of the battery is also a variable that make's winter starts harder.








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