Here is something else to think about. It may not pertain to most but for guys that are looking to get the best out of their engine even if it's in our minds try this...
Everyone is talking about viscosity and the drag on the
oil pump. How about this..
Oil gets whipped up and fills with tiny air bubbles. Those air bubbles unlike
oil are compressible. Take 50W oil for instance, the air bubbles will stay in suspension alot longer than in 30W oil. Now take the column of oil in a hydraulic lifter especially if you are running the plunger close to the top with .150" of oil. What happens at hi rpms? and strong spring pressures? COMPRESSION/COLLAPSE and loss of power. Lighter oil releases the air bubbles quicker. Now of you have a cheap lifter and the clearances are not built to close tolerances the oil will leak past the body and the plunger causing loss of lift and possible collapse. Thats when 40-50w oil is needed, just like loose bearing clearances. It depends on how your engine was built and the parts used as in the lifter case.
I know this is just usless information to most but is something to think about for some.
I run 175lbs. on the seat and 450lbs on the nose of my hydraulic cam. Oil recommendation for my lifters is 30w and I run them down pretty low so there is not alot of oil in the body. This is they way I like to run my engine and it has good power right to the intended rpm limit and beyond.
Run the lightest oil your engine needs and NO more-- whatever that my be.
warning___this is just my opinion not intended to knock anyone's theory of fact, for added conversation........ and it is not baseless.