What engine oil for a EJ-DE engine? And does most run 91 or 95 fuel?
I run 98 and a 10/40 is my preferred weight in oil. I only ever run 91 on any dai if I have no choice at the servo.
If it has done a lot of K’s E10 or 95 for the higher octane as it may have a lot of carbon in the combustion chamber. You can get hot spots, higher comp and detonation. I’d use 5w30 as per factory recommendation. OEM and piston/ring manufactures spec oil based on cyl hone pattern. Too thick and the rings scrape the oil away without lubing properly. Oil pressure keeps the lub in the bearings and thicker oil not needed. Lower oil weight means more power. Look at the research from oil companies like Amsoil (who invented/found full synthetic) or Joe Gibbs race oil, the trend in top race engines is lower weight oil - more power and this has a lot to do with thinner rings, cnc home and reduced friction. A lot of bikes still run heavy weight oil but that is due to needing to lube the gearbox and the extra heat. That said, if you are in a hot climate then a 40wt might be okay.
5W30 is spec as has been said by Gormsby as well.
Personally I only run 98 (E5). I get way better mileage on it and in theory it’s better for the engine and fuel system as well…
And this might be placebo but it seems to have better throttle response.