It's never easy to choose the best viscosity for your engine. Ideally you want the thickest oil film, lowest friction, and best cooling. But these three variables can be reversely correlated, making the job difficult.
First, it's important to understand the difference between SAE viscosity and apparent viscosity. Apparent viscosity is as a function of temperature, as well as a function of shear (fast sliding action). (1) Higher the temperature, less the apparent viscosity. (2) More the shear (fast sliding action), less the apparent viscosity because of the temporary shear (viscosity loss) of viscosity-index-improver polymers under mechanical shear (fast sliding action).
Minimum oil-film thickness (MOFT): More is always better to reduce wear.
MOFT increases with apparent viscosity.
MOFT increases with engine RPM.
MOFT decreases with engine power output.
Friction: Less is always better to reduce wear and increase fuel economy.
Friction increases with apparent viscosity.
Friction decreases with good friction modifiers and good detergents.
Cooling: More is always better to decrease wear.
Cooling decreases with apparent viscosity, as apparent viscosity increases friction and decreases flow.
To summarize:
High viscosity: Potentially good MOFT for good wear protection. But then the sliding surfaces run hotter due to more friction and less flow, which is bad for wear. Also, oil temperature is higher, which may decrease the apparent viscosity and the MOFT. More friction is also bad for wear. Less oil flow is also bad for oil cooling and wear.
Low viscosity: Potentially bad MOFT, with the risk of increasing engine wear. But then the sliding surfaces run cooler due to less friction and more flow, which is good for wear. Also, the oil temperature is lower, which may increase the apparent viscosity and the MOFT. Less friction is also good for wear. More oil flow is also good for oil cooling and wear.
So, how do you optimize your viscosity. Sometimes xW-20 vs. xW-30 can make a difference, as even a 20% increase in viscosity can increase the low-RPM safety margin by 20% -- oil-film breakdown happening when the RPM falls to 1200 RPM instead of RPM falling to 1500 RPM. (Oil-film breakdown happens at low RPM and high power.) But then better cooling and less friction with lower viscosity can actually increase the oil-film thickness by cooling the contact surfaces and increasing the apparent viscosity. If you have an antique engine, chances are that your clearances are so large that choosing a low-viscosity oil won't increase your oil flow and cooling but increase wear. Also, with diesel engines, you want thick viscosities because abrasive soot particles cause a lot of wear and you want as high a MOFT as possible to separate the contact surfaces from grinding against the soot particles. But with a modern gasoline engine, how do you find the optimal viscosity for best wear protection?
First, it's important to understand the difference between SAE viscosity and apparent viscosity. Apparent viscosity is as a function of temperature, as well as a function of shear (fast sliding action). (1) Higher the temperature, less the apparent viscosity. (2) More the shear (fast sliding action), less the apparent viscosity because of the temporary shear (viscosity loss) of viscosity-index-improver polymers under mechanical shear (fast sliding action).
Minimum oil-film thickness (MOFT): More is always better to reduce wear.
MOFT increases with apparent viscosity.
MOFT increases with engine RPM.
MOFT decreases with engine power output.
Friction: Less is always better to reduce wear and increase fuel economy.
Friction increases with apparent viscosity.
Friction decreases with good friction modifiers and good detergents.
Cooling: More is always better to decrease wear.
Cooling decreases with apparent viscosity, as apparent viscosity increases friction and decreases flow.
To summarize:
High viscosity: Potentially good MOFT for good wear protection. But then the sliding surfaces run hotter due to more friction and less flow, which is bad for wear. Also, oil temperature is higher, which may decrease the apparent viscosity and the MOFT. More friction is also bad for wear. Less oil flow is also bad for oil cooling and wear.
Low viscosity: Potentially bad MOFT, with the risk of increasing engine wear. But then the sliding surfaces run cooler due to less friction and more flow, which is good for wear. Also, the oil temperature is lower, which may increase the apparent viscosity and the MOFT. Less friction is also good for wear. More oil flow is also good for oil cooling and wear.
So, how do you optimize your viscosity. Sometimes xW-20 vs. xW-30 can make a difference, as even a 20% increase in viscosity can increase the low-RPM safety margin by 20% -- oil-film breakdown happening when the RPM falls to 1200 RPM instead of RPM falling to 1500 RPM. (Oil-film breakdown happens at low RPM and high power.) But then better cooling and less friction with lower viscosity can actually increase the oil-film thickness by cooling the contact surfaces and increasing the apparent viscosity. If you have an antique engine, chances are that your clearances are so large that choosing a low-viscosity oil won't increase your oil flow and cooling but increase wear. Also, with diesel engines, you want thick viscosities because abrasive soot particles cause a lot of wear and you want as high a MOFT as possible to separate the contact surfaces from grinding against the soot particles. But with a modern gasoline engine, how do you find the optimal viscosity for best wear protection?