I agree with OnebyOne about soaking thread in mineral oil.
As soon as my thread starts breaking I know it's time to
give it the oil treatment. Makes a big difference in my
sewing. I also change needle on a regular basis.
You can watch this Youtube video if you are interested.
https://www.youtube.com/watch?v=-xRjSAbl_NU

I prefer Superior Thread's, 50 weight, "So Fine". It's strong, but thin enough to blend in better than a 40 weight.

I quilted my son's quilt with Gutermann cotton, and the threads are all breaking.

I've decided I like to FMQ with polyester. It just behaves better.

I use Isacord 40 wt thread on my longarm with no problem. My dealer has every color they make, over 400. I've also used Glide and Aurfil without any problems. I use an 18 needle.