[Bob Bidonde]

1. All of the sludge filled engines I have disassembled used non-detergent single viscosity motor oil.
2. The higher the oil's viscosity, the more energy it takes to pump it, and it flows slower due to the effect of gravity. Oil to the crankshaft bearings and camshaft bearings flows via gravity through small holes.
3. Synthetic oils have a higher operating temperature and a higher squeeze-out resistance. Oil pressures in the main bearings is THOUSANDS of psi. Also valve lifter to camshaft faying surfaces benefits from higher squeeze-out.
4. The cost of synthetic oils have come down to be competitive with non-synthetic oils.