It used to be that in that if you ran conventional you ran 15w-40 and if you ran synthetic then there was no 15w-40, so you ran 5w-40. Typically for me, it would be Rotella, T4 vs T6 (since you don't know how much of each there is in a synthetic blend if I felt like saving some money I would get a gallon of one and 2 of the other and mix them rather than buy the T5).
Lately, I've noticed 15w-40 T6 and other full synthetics showing up and as I'm about due or an oil change I've been wondering if there is an advantage in using 15w-40 over 5w-40 assuming that they are both the same type and brand oil (looks like some of the same in showing up in the other brands like Delvac...)? Since at operating temp they should both be about the same, but the 5W should be better on cold starts I'm wondering why Shell and others are suddenly coming out with a 15W40 synthetic. I know the conventional answer is that you should run as tight a "spread" as possible for your temp range because it takes more viscosity modifiers to make a larger spread which makes the oil more susceptible to shear damage, but I thought that the way synthetics are made that isn't an issue?
FWIW, this is in a 5.9 Cummins Ram that is likely to see temps 0-100*F during the next oil change interval, though 90% of that will likely be in the 50-90* range (here in the DC metro area we will get cold nights getting down into the 0-5* range once or twice a year, but we've had something like 30something days straight getting over 90* so far this summer and almost 50 that it hasn't gotten below 70*F)
Lately, I've noticed 15w-40 T6 and other full synthetics showing up and as I'm about due or an oil change I've been wondering if there is an advantage in using 15w-40 over 5w-40 assuming that they are both the same type and brand oil (looks like some of the same in showing up in the other brands like Delvac...)? Since at operating temp they should both be about the same, but the 5W should be better on cold starts I'm wondering why Shell and others are suddenly coming out with a 15W40 synthetic. I know the conventional answer is that you should run as tight a "spread" as possible for your temp range because it takes more viscosity modifiers to make a larger spread which makes the oil more susceptible to shear damage, but I thought that the way synthetics are made that isn't an issue?
FWIW, this is in a 5.9 Cummins Ram that is likely to see temps 0-100*F during the next oil change interval, though 90% of that will likely be in the 50-90* range (here in the DC metro area we will get cold nights getting down into the 0-5* range once or twice a year, but we've had something like 30something days straight getting over 90* so far this summer and almost 50 that it hasn't gotten below 70*F)