I don't remember having to adjust the oil shutters very often in flight in the BE-18.
The oil coolers on a Twin Beech are not equipped with an automatic by-pass (thermostat). In flight, the oil temperature is controlled manually with the cooler inlet shutters.
The oil coolers do have individual manual by-pass controls intalled that are intented to be used on a cold start until the engine comes up to operating temperature. [Note to original poster: make sure the engine is at idle when you open the by-pass valve! Otherwise, warm oil in the system at high pressure will hit the congealed oil in the cooler and could cause the cooling radiator to rupture].
