This is another interesting article on oil viscosity and bearing clearance. https://blog.k1technologies.com/bearing-clearance-and-oil-viscosity-explained
There are several interesting take-aways:
1) The typical rule that most engine builders follow is that bearing clearance establishes the viscosity of the oil to use within a given oil temperature range.
2) Generally, the tighter the bearing clearance, the lower the viscosity the engine can use while wider clearances require thicker oil.
3) A bearing’s load carrying capacity is directly affected by clearance. As the clearance is tightened, load carrying capacity increases.
4) Common sense dictates that bearing clearance directly affects oil flow with predicable results. Reducing the clearance will increase the restriction to flow and reduce the volume of oil past the bearing. Because of this reduced flow, the localized bearing temperature will increase. If this temperature exceeds the oil’s thermal stability limit, the oil will begin to oxidize and break down, reducing its ability to lubricate.
So maybe, just maybe the manufacturer knows what they're talking about when they recommend a 5W-20 oil.
There are several interesting take-aways:
1) The typical rule that most engine builders follow is that bearing clearance establishes the viscosity of the oil to use within a given oil temperature range.
2) Generally, the tighter the bearing clearance, the lower the viscosity the engine can use while wider clearances require thicker oil.
3) A bearing’s load carrying capacity is directly affected by clearance. As the clearance is tightened, load carrying capacity increases.
4) Common sense dictates that bearing clearance directly affects oil flow with predicable results. Reducing the clearance will increase the restriction to flow and reduce the volume of oil past the bearing. Because of this reduced flow, the localized bearing temperature will increase. If this temperature exceeds the oil’s thermal stability limit, the oil will begin to oxidize and break down, reducing its ability to lubricate.
So maybe, just maybe the manufacturer knows what they're talking about when they recommend a 5W-20 oil.