I have an '03 EP3 and I am having a debate with some fellow Si owners about which oil to use. After reading quite a bit on this forum I feel like I am protecting my engine by running 5w-20 but they keep arguing that Honda only specs 5w-20 for fuel mileage. My engine (K20A3) has the same bore and stroke as the engine found in the RSX-S but has a different head, lower compression, lower redline, and a different style of VTEC. Their argument is basically that because the RSX-S and my engine share the same bore and stroke and the RSX-S specs 5w30 that 5w30 will protect better. I feel that the difference in redline (mine is 1300rpm lower), head design, and type of VTEC (mine, although labeled i-VTEC is really more a type of E-VTEC) will mitigate any similarity between the bore and stroke. I did run 5w30 M1 last OCI but didn't notice any difference except for slightly lower fuel mileage. Are either of us right and is there really any way to tell other than running a UOA?
Jon

Jon