An oil thread...perhaps
Why - would you use 15w40 over 5w40 (beyond break in) if you had the choice?
As I understand it, the first number in any given multi-grade oil is the effective cold operating viscosity index and the second number is the effective hot (all the way warmed up + ambient temp) operating temp. So, if you're not wanting to change oils from summer to winter, and 5w is a lot more universal of a cold weight than 15w, or 20w for that matter, why would 15w or 20w ever be a better alternative than 5w in a liquid cooled engine that is subject to many cold start up situations?
There are some summer days here in East Texas where it's 80 or so in the morning, but in the winter it does dip below freezing on occasion, and as I understand it, 5w is plenty fine for either temp. So again, how is 15w, or 20w ever a viable alternative? I'm not saying it never is, I'm just truly curious.