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Years ago (maybe 25) Ford dropped the spring pressure on their relief valves, so max oil pressure was reduced by almost 50%. That alone netted them an estimated .5 MPG on the EPA cycle. They also found that modern oils work just as well or better using the philosophy of higher flow versus viscosity as had been done decades ago. The variable displacement pumps are another step in reducing oil pump drive losses.
I would just run the lowest viscosity oil recommended for your engine, but beware of manufacturer specific limitations. I know Nissan still recommends 5w-30 weight due to cam lobe wear issues, where they have finer surface finishes that need that viscosity.
regards
mech
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