Dlduchon, switching fuel and idling are the reasons for the mileage drop. Look again at those winter fronts on the trucks and you'll notice something: Most newer winter fronts have a hole in the center. Keeps some air moving (parts of the engine, like exhaust manifolds, are air cooled) and located in front of the center of the fan (keeps a balanced load on the shaft bearings). I use a paper beer or soda case because it's already painted. Cut out to fit and attach with 20ga solid core electric wire so that the twist is inside the hood. Easy, fast, and cheap. Some oil coolers have thermostats, some don't. If you need a winter front for the radiator then it won't hurt anything if the oil cooler is covered.

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daBenz - 1970 220D