0W-20 isn't a problem. If it didn't boil so easily and you never shut the engine down, plain bearings would be perfectly fine with water rather than oil. Once an engine is running the journals big-ends etc. are kept apart from each other by the hydrodynamic wedge formed by the relative motion of the parts - how good the oil is as a lubricant is then irrelevant. The lubricative properties of the oil are really only a concern at the top of the stroke where the oil film can get burned away and as the nose of the cam reaches the follower - that's when the fluid "wedge" spills away and film strength becomes a concern - and of course during start up where there is metal to metal contact. Roller bearings wouldn't be so tolerant of running in water!
Modern oil isn't actually that great as a lubricant, a large part of the makeup is detergents (which offer nothing for lubrication - quite the opposite), viscosity enhancers (ditto) and chemical potions designed to suspend small particles in the oil rather than let them settle to make sludge. Meaning that over time your oil becomes an abrasive solution. Lastly there little or no zinc (damages cats) the best last line of defence if metal to metal contact does occur. Oil of 50+ years ago would be far, far better as a lubricant - but you'd end up with oil sludge which would kill the engine in no time.
The reason for the above is extended change intervals. The chemical stability of the oil is important to protect seals. Oil filters are designed NOT to filter too efficiently or they'll become clogged over 20K miles - so small particles bypass the filter and are suspended in the oil. If you want your engine to last - change the oil regularly - regardless of the ridiculous modern intervals stated - they can only get away with those intervals because modern engines are so good.