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The SAE (Society of Automotive Engineers) viscosity (grade, not weight) is, by definition, thickness (resistance to flow). It’s the time in seconds for a given volume of oil to pass through a sized opening, not the oil’s lubrication properties.
A single SAE number (without the “W”), or 2nd number of two is the viscosity at high oil temperature: 212° F, or 100° C.
A number before the “W” (e.g., 10W30), or the 1st number of two (e.g., 10-30) is the winter rating (not “weight”), indicating the viscosity at low oil temperature: 23° F, or -5° C.
“Straight” (single viscosity) 40 grade not only reduces cold cranking speed, but gives no additional protection or safety to the crankshaft. It simply does not reach the bearings quickly at low temperatures, and does not flow through the bearings fast enough to cool them. Bearing (and crankshaft) failure is far more likely to result from too much viscosity than from too little, as long as oil pressure is acceptable at all engine speeds.
Cold 50 grade oil flows through a bearing as fast as asphalt flows through a drinking straw.
Multi-viscosity detergent oil provides oil pressure more quickly after start-up than single viscosity. The detergents and dispersants transport the dirt & debris into the oil filter for removal. “Mobil 1” 10W30 or 40 is excellent for all purposes.