An excellent post and suggestion for an oil for a 60's Chevy 348 by 440Magnum, has caused me to look at viscosity from a new (to me) perspective.
5th post in this thread:
https://bobistheoilguy.com/forums/threads/modern-oil-in-rebuilt-old-motor.132089/
Instead of focusing on bearing clearances and oil temperature for choosing an oil viscosity, maybe we should consider the flow volume through the system. Pressurized lube systems are a controlled leak. Think about where the oil goes after it exits the pressurized system and continues to lubricate and cool until it returns to the sump. Lower visc means faster leakage and more oil moving through the system, possibly increasing splash oiling and cooling.
A factor is how much cooling capacity is available to the returning oil before it is pressurized again. Some engines may have a more limited ability to cool oil and the extra flow wouldn't do as much good. Maybe this is part of the reason some see lower oil temps with lower visc oils. The engine likes greater flow and has the ability to cool it.
We don't talk much about an optimal flow rate of oil passing through the pump. Extremes are pressure not being built, and a system in pump relief bypass mode. We want the system to be between those extremes, and one of the ways to keep it there is with optimum viscosity for that engine.
Some engines like lots of flow and others don't have the capacity to utilize it, due to pump, gallery design, oil cooling ability, or they like more limited splash. Oil return ability is also a factor, if you can't get it back to the sump quickly, you would want less that isn't under pressure or in the sump
Maybe when Ford says to stay with 5w30 for the 4.0 V6, they don't make this rec based on the bearings needing a thicker cushion, or the oil temps being higher, it might just be the upper end doesn't return oil well.
5th post in this thread:
https://bobistheoilguy.com/forums/threads/modern-oil-in-rebuilt-old-motor.132089/
Instead of focusing on bearing clearances and oil temperature for choosing an oil viscosity, maybe we should consider the flow volume through the system. Pressurized lube systems are a controlled leak. Think about where the oil goes after it exits the pressurized system and continues to lubricate and cool until it returns to the sump. Lower visc means faster leakage and more oil moving through the system, possibly increasing splash oiling and cooling.
A factor is how much cooling capacity is available to the returning oil before it is pressurized again. Some engines may have a more limited ability to cool oil and the extra flow wouldn't do as much good. Maybe this is part of the reason some see lower oil temps with lower visc oils. The engine likes greater flow and has the ability to cool it.
We don't talk much about an optimal flow rate of oil passing through the pump. Extremes are pressure not being built, and a system in pump relief bypass mode. We want the system to be between those extremes, and one of the ways to keep it there is with optimum viscosity for that engine.
Some engines like lots of flow and others don't have the capacity to utilize it, due to pump, gallery design, oil cooling ability, or they like more limited splash. Oil return ability is also a factor, if you can't get it back to the sump quickly, you would want less that isn't under pressure or in the sump
Maybe when Ford says to stay with 5w30 for the 4.0 V6, they don't make this rec based on the bearings needing a thicker cushion, or the oil temps being higher, it might just be the upper end doesn't return oil well.
