I'm a bit confused after looking at the specs of these two oils in terms of cold weather performance. They both seem to be very close in terms of how they pump at low temps, as their CCV at -25c is almost the same (4500 for the 10w30, 4877 for 15w40) but yet their viscosities at 40c are drastically different (15w40 is 108-111cst, while 10w30 is much thinner at 62-70cst) So how can the 15w40 flow almost as good at low temps when it is so much thicker on the 40c viscosity test? Different base oils perhaps? When I am done my long term test on the 10w30 I may just go to the 15w40 to compare, as it seems like it can handle the cold weather we get here (which as I've said before rarely goes down below 0F) I've heard it mentioned before that the 15w40 Schaeffer would actually qualify as a 10w40 and based on the numbers I believe it. Would you guys recommend the 15w40 for my daily driven and drag raced 95 LT1 Firebird? What benefits would I get with the 15w40 compared to 10w30 in terms of the additive package and long drain capabilities? I notice the 15w40 has a TBN of 10.0 compared to 7.1 on the 10w30, how are they achieving the higher TBN on the 15w40 and does it maintain it's TBN as well as the 10w30 does?