The reason manufacturer's in the auto industry have gone to lower viscosities has to do with fuel economy ratings; a less viscous fluid requires less power to pump, therefore reducing parasitic power loss. They have been able to do this without sacrificing durability due to the many advances in materials, lubricants, and manufacturing tolerances over the past 30+ years. Modern engines running 5w20 will easily outlast the old iron running "heavier" oils. Think about it; we changed oil in our cars every 2000 miles in the old days, and 100,000 miles was pretty much the top end of the useful life scale (many engines needed major work long before that). Current automotive engines blow right past that 100,000 mile mark with 5,000 to 7,500 mile oil changes and very little else in terms of maintenance (change spark plugs one time, air filters every 20,000 or so).