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Yes, a thinner viscosity oil would reduce parasitic drag a bit, not sure if it would be noticeable. I'd go with a 10w30 during the winter. With the life that engine has had I assume the bearing tolerances have opened up a smidge and I wouldn't want to risk going much thinner than that.
A roller cam retainer plate bearing and roller rockers would also slightly reduce parasitic drag. Lots of little differences can add up to a big difference!
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'99 Crown Victoria
'97 Impreza 2.2, 5-speed
'98 Impreza Outback Sport 2.2, 5-speed
'05 Outback 3.0R (wife's)
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