I plug my sdl in every night as it reduces wear on the engine with the cold weather start ups and the heat comes much faster. The engine block heater is well worth the cost of about 100. I have tried the magnetic engine heater on the oil pan and it did not provide enough heat. My car sits outside every night. I also live in Minnesota.

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91 350 SDL
91 300 E
85 300 D (sold)
79 300 CD (sold)
91 300 CE