15w40 vs 5w40 ?
 

What's the difference really? Do we need it to be thinner and flow better on startup or thicker? The chart in manual has both but only 5w40 covering the whole range of temps?

15w-40 is thicker and is slightly better for higher temps.

Real world application? Run whichever you want. The difference isn't that great. Most people run 15w-40 because it is more readily available.

Alright. Thanks for your help.

i think the diesel oil has more adatives. harley davidson says in service manual to run harley oil or diesel oil no auto oil.

Just make sure it's diesel approved. 5w will flow a little better on cold starts. But I run 15w and it hits -30F here, I've never had any problems with pressure.

A thread was just started about this over on CF too. Turned into a fight lol.

Heavier weight in the summer, lower weight in the winter.

The reason is obvious; tempurature. Sure, 5W40 will handle the high temps but I'd rather have a bit more protection.

Both run the same at operating temps as a 40 weight. Startup is where most engine wear can occur so using a 5W in cold temps should theoretically help with this. As mentioned, a 15W can work to pretty low temps.
You're correct in that 5W are easier to find these days. Many companies have gone away from the 15 as 5 will serve all purposes and saves them from having to make several grades.
Being in Georgia, a 15 will easily serve you purpose.

Diesel oil has higher anti wear properties than gasoline oil. It handles high temps pretty good to. However, the biggest reason to NOT use gasoline oil is because of the friction modifiers it has in it. They were designed to improve fuel economy by creating less drag.Good for cars bad for wet clutch systems. The engines and transmissions in most motorcycles share the same oil.

I'm not much of a Harley guy, so I wouldn't know which ones share oil and which ones wouldn't.
Not around here. 15w-40 is by far the most prolific.

Yeah, I've personally never seen 5w-40

walmart has it in rotella t in a blue bottle