Yes, using a thinner (not necessarily synthetic) oil (5W-20, 0W-20, etc ...) will provide less drag/resistance and possibly improve fuel economy by a measurable amount.
Most cars that currently spec 5W-20 were designed with 5W-30 in mind. So, it stands to reason that many engines that currently spec 5W-30 can be switched over with no adverse effects ... just a slight increase (0.25 - 0.50) in measured fuel economy. If you search the UOA section of BITOG, you can find others that have already gone this route and had a sample tested for signs of increased wear. I haven't seen all these results, but I have yet
There are two other factors that make me comfortable with this idea:
1) With any interval past 3,000 miles, most XW-30 oils shear down to a XW-20 anyway. With the extended intervals recommended by most manufacturers these days, many engines are operating with a 20 weight oil in the sump.
2) When some manufacturers (Ford and Honda come to mind) switched over to 5W-20, they made sure the oils were made with more Group II+ or Group III base stocks for added stability and they increased the barrier anti-wear additives such as moly and boron. UOAs of these 5W-20s show this clearly.
Now that ZDDP levels in SJ, SL, SM oils are lower than their predecessors, the blenders have discovered that adding moly and/or boron can make a very high-performance, low-wear oil.
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--- Bror Jace
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