Almost no one sells 40 cetane. 40 is the minimum and I would guess no one sells fuel at the rock bottom minimum. 50 is the Euro standard IIRC. But the cars here run just fine on pump fuel. Higher cetane has diminishing returns at some point vs cost. If your car runs fine on regular pump diesel, run it. If you don't mind paying the extra cost of "premium" diesel or cetane additive then use it. I have tried additives in the past and saw no noticeable improvement. So I don't use any. I do buy fuel that has a bio component to it for lubricity purposes, but almost all pumps here have that. Cold weather is the one use for additives that I think is really needed. But being that I live in Texas not really a factor for me.