Unneeded stress on oil pumps in cold ambient that can lead to premature pump wear and eventually oil pressure loss , more fuel used during the warm up phase , hydraulic lifters engineered for 30wts and below that don't operate will full efficiency with incorrect oil , higher cylinder wear from cold starts and a number of other things are some of the likes oil analysis cannot pick up that leads people to believe these 15w-40's do better than lower VI PCMO's in modern gas engines . There's more to it than meets the eye . Alot more .
Even the 5w-40 diesel oils are a bit too much for most automotive gas engines . They are around 100-110 @ 40c whereas a nice PCMO 5w-40 will come in around 75-85 @ 40c .
Motorcycles in hard use will smitherine those diesel synthetic 5w-40's also .
This is a never ending debate . The thicker is better thing ... the doping of well built oil with additives ... ahhh forget it . Have other things to do
