Modern engines will run a LONG time on 10w30 mineral oil. Synthetic IS better and is less susceptable to acid formation, therefore the longer calendar life. It also extends the milage life. Synthetic rear axle oil (90w) is miraculous stuff! Ford actually raised the GVW rating based only on oil change.
SOme years ago, GM had a bulletin out that was hard on 10w40 oils. IIRC the additive package that gave the better hot viscosity (40) added dramatically to deposits in ring grooves and other places.
An oil film of adequate strength is all that is required. Viscosity is vastly over rated. Most top end drag cars use 0w oil. In the Top Fuel engine I used to work on we used stright 30W - mineral oil. 10 years ago, Indy cars used 30w Pennzoil. Mucho snake oil and hocus pocus in the oil commentary business!!!!!!!
You should see the watery thin oil we use in the helicopter. Full synthetic, very thin, (looks like 5w) change at 100 hours (~10,000 miles). Engine is worth $100k+ for OH, expected life (TBO) is 1800 hours, nearly 180,000 miles! (And that is because of turbine wheels, NOT bearings)! N1 turns 58,000 RPM at 100% power. Thing doesn't even use an oil filter, just a fine mesh screen about 1" diameter, 2 1/2" long. Wash and re-use IIRC.