15W-50 is fine. Synthethics flow better at low temps so in your climate there is no need for a thinner oil. The harm in the short trips will not be from too thick oil but rather carbon build up and possible condensation in the crankcase due to the engine noy getting to full operating temp. Do yourself a favor and at least once a week take her out for a good, spirited 20-30 minute drive to let everything get nice and hot and blow the carbon out.

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LRG
1987 300D Turbo 175K
2006 Toyota Prius, efficent but no soul
1985 300 TDT(130K miles of trouble free motoring)now sold