So I use 20W-50 motor oil in my Dyna Wide Glide, but a buddy gave me a gallon of 10W-40. Is that safe to use?

 

Put the 10W-40 in your car.

 

If you're going to be riding in below 40° ambient temperatures HD does recommend 10W-40 but unless you are, it's a poor choice for the engine.

You can use 10W-40 in the primary, though. I use Mobil 1 Racing 4T 10W-40 synthetic in the primary and it works well. The 4T is JASO rated so no issues with the clutch...

 

thanks for tips - turns out my buddy uses 10-W40 so I'll give it to him....

 

I've run 15W-40 diesel oil in my bike in summer heat and it works just fine. Don't notice any difference from when I ran 20W-50.

Since you're in Norway I wouldn't be too concerned about a lighter weight oil.

 

As always I am to late with my answer, but that saying I to use I use Mobil 1 Racing 4T 10W-40 synthetic in the primary. I think you are talking car oil so yes use that in a car.

 

I'm just across the North Sea from Norway and the weather is well above 40 degrees there at present, only a bit cooler than here in the UK. I only use 20-50 in my bikes in summer and no longer ride in winter, when temps might warrant 10-40.

 

I used to work for GEC so have been to your country a few times.
It still isn't all that hot. I'm running 15W-40 right now and temps are in the 90's.

 

Run the 10-40 in your primary.D

 

I would use the 10w40 in the car