G,
The question of what's better for cold starts conditions - thick or thin oil?, is a simple one. the answer however depends upon many things within the dynamic nature of the engine, and what one may put in order of concern.
For ease of starting, I would use a light weight oil that allows the engine to crank relatively easily, and flows to vital engine parts quickly. Now with the last part of ensuring quick flow, does that mean that whatever oil that's left from its last period of operation isn't going to protect? Well, the oil becomes fairly thin when it's up to it's normal operating temperature, and I bet it took some time for things to cool down. Having said that, I'm not sure there's much oil in place. One must also consider that now that the oil has thickened due to being of a cooler temperature, it's appearent increase in viscosity will result in a related increase in it's shear strength. Will there be enough to generate a hydrodynamic film/fluid wedge, and to delay shear through until oil pressure builds and oil replenishment ensues???
With an engine having hydralic lifters, I would personally like to ensure that the clearences between lifter tappet and cam lobe remains in check, rather than hearing a sewing maching run under the hood. The shock loads upon the tappet in the bore, the cam profile, and cam bearings leads only to bad senses, though little harm might actually come of it (as an owners manual may read, "this is completely normal"). Harm none the less, yes?
Beyond the cold start situation, engine operation there after may be of limitted demand until the oil itself warms up. I recall when priming an engine's oil system by driving the pump by a drill in 70 degree weather and using 10w-30 oil, the drill was fairly boged down. Just going to prove that until the oil warms-up and resultingly thins, a good amount of energy is going to be consumed in just trying to get the oil flowing. And with such restrictive clearences, oil pressure is likely to build even at idle, past the point where the oil system's pressure relief valve comes into effect. Just how much of the oil that's being pumped is actually going through the oilways rather than out the bypass circuit is beyond me.
Lastly, let it be noted that "heavier weight oil is best suited to low speed, high load operation, where as lighter weight oil is best suited for high speed (low load?) operation." All engine's oilways being designed accordingly to enable quantity distribution accordingly by designed requirements.
Where might that leave one? -
