guys, is there any reason a 15/40 would be preferable over 5/40?
Almost everyone associates 15/40 with diesel.
Let's assume no extreme cold, but plenty of hot.
I know in high temps it's the 40 one worries about, but
are there compromises in the oil to accomplish this?
Or is one oil simply formulated more "completely" to accomplish a wider variety of tasks?
Realistically, and decent type of either grade should give you good service. I suspect you understand the cold benefits.
In terms of nuances, hot, I checked Amsoil to compare two synthetic versions of those weights. (What I'm trying to do is compare to equivalent base oils, as many 15W40's tend to be conventional). Long story short, the equivalent 15W40 has about a 12degC higher Flashpoint.
That higher Flashpoint should give you a
slightly lower oil consumption under
extended high temperature runs, esp. at high loads. My guess - if you were pulling a loaded cattle trailer all over Texas in the summertime, AND the motor was oil tight (no leaks) you might see the difference. Most people don't pay that much attention though.
Conventional oil, in a given weight grade, tends to have a lower Flashpoint than synthetic. If you know your road vehicle well, you can see this tendency (
slightly higher oil consumption with conventional) esp. during the summer months.
Way back, the standard wisdom was that you didn't want too much of a spread between the top/bottom viscosity #'s. The historical reason was that the techniques used to get a wide spread weren't as good as what is the practice today..... long way of saying that modern oils (Conv. and Synth.) are better than they used to be.
That said, engine use and maintenance practice can be a factor. Assuming I didn't have to deal with cold temperatures, for somebody that changes their tractor oil every 5 years whether it needs it or not, I'd probably want to see 15W40 in there - but that is an extreme situation - most TBNers don't let things go that long.
Rgds, D.
