Ordinarily I wouldn't comment on someone's engine oil choice, simply because EP3 owners seem to have fixed opinions WRT their engine oil choices, as reflected by this board. That said, IMHO back-specing an EP3 (or any Honda, for that matter) to a lighter weight oil in anticipation of a potential gas mileage increase may not be such a good idea.
As I'm sure you know, Honda recommended 5W-20 weight oil to ensure its fleet-wide MPG average would meet (and exceed) an EPA standard. Honda did that to receive millions of dollars in Federal funds (tax incentives) and to avoid similar fines for failing to meet those standards. That weight oil is fine if you drive conservatively, as most Honda owners do. (Auto tranny, 2K-3K rpm, etc.) However, as you also likely know, HMC continues to recommend 5W-30 weight oil for all of higher performance K-series motors (and 10W-30/10W-40 for the S2ks). Only the latter weights are HT/HS HTO-06 certified or capable of meeting that standard. Also, going from 26 or 28 mpg to 30 mpg via an engine-oil weight change -- i.e., 5-10% mpg gains -- would be considerably more than what HMC has been able to achieve in the past 10 years with regearing and compression ratio changes. As an alternative, short-shifting will net you what you want without the potential for camshaft galling or, eventually, piston slap.
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