Why 0w20 oil is death to red bearings?

Yea your basically adding cooking oil (but obviously not cooking oil) its very thin, but oil is not only for lubrication, yes it wears them out, but oil runs through the system so fast its going to relube it very quickly anyways. Oil also removes contaminants, heat, and distributes the heat energy evenly.

If you look at all different types of oil your going to see different colors, and some are going to look sludgy compared to others. If your engine is tuned (like a mustang) to run super hot and fast, you need a medium base, that will cool it while lubing it up.

Now some cars cannot move oil that fast like the mustang, so you need a thinner batch while still maintaining the heat and pressure (some engines are bigger, some get hotter etc) this is where thinner oil comes in, but faster flow rate for faster heat reduction. If your oil gets to hot, but can't move quickly enough, it could overflow, or cause enough pressure to crack your engine.

Oil plays a big role yes, but adding the wrong oil can damage your engine very badly, but adding the right oil could make it even better. (It's best to follow what the manufacturer says, but if you want something better make sure your engine can handle it. You don't want to blow a gasket or anything.

The answer to your question is no, 0W-20 oil is NOT death to red [sic] bearings. I presume it is a typo for rod bearings.

Oil viscosity is dictated by bearing clearance. If 0W-20 is used in an engine with .001-.002 inch clearances (which should use 30 instead of 20 weight) the bearings will pass too much oil and low oil pressure may result, but at least the bearings will have plenty of oil delivery. If 5W-30 oil is used in an engine with .0003-.001 inch clearances, (which should use 20 instead of 30 weight) the bearings will be starved for oil; the pressure will be easier to maintain but the oil will not flow properly through the tight tolerances. It is generally safe enough to use a lighter grade (like 20 instead of 30) or a slightly different winter viscosity (like 10W-30 instead of 5W-30) but going to a higher viscosity is not safe. If the engine calls for 0W-20 do not use anything heavier than 20 weight, regardless of miles on the engine or ambient temperature. That is extremely bad advice.

You have no clue about oil or engines.

Why the question mark? Your making a statement not asking a question.

See below- cold startup is one of the most detrimental of times for rod bearing - oil has dripped off the components for a long time - but YOU Believe what you want to believe - I believe I will stick with the manufacturers recommendations.

In a 10w-40 for example the 10w bit (W = winter, not weight or watt or anything else for that matter) simply means that the oil must have a certain maximum viscosity/flow at low temperature. The lower the "W" number the better the oil's cold temperature/cold start performance.

The 40 in a 10w-40 simply means that the oil must fall within certain viscosity limits at 100°C. This is a fixed limit and all oils that end in 40 must achieve these limits. Once again the lower the number, the thinner the oil: a 30 oil is thinner than a 40 oil at 100°C etc. Your handbook will specify whether a 30, 40 or 50 etc is required.