I switched from Valvoline 5w30 to M1 Delvac 5w40 in my '01 prelude...fuel economy improved by maybe 5% (overall improvement was from 27 to 31 mpg, but probably 1-2mpg was the non-MTBE fuel used here). Personally, I wouldn't run 5w30 unless you live somewhere where it's cold...I heard the main reason why more carmakers spec 5w30 these days is that's what they use to boost the CAFE numbers in EPA tests, and the gov't requires the same weight of oil be spec'd and sold at the dealership. For example, in the case of toyota trucks, toyota specs 5w30 in the US, yet heavier weight oils elsewhere. Personally, I wouldn't go below a 10w30 during the summertime, maybe a 5w30 if you live in the NorthEast for winter.
[ July 17, 2003, 11:52 AM: Message edited by: James Chow ]