Buy a 5/16" solid rod and take 2 minutes and grind a hex on one end and your good for life. Cheaper than buying a paint stir or anything else. I made one 20 years ago out of a 5/16" piece of tubing. I've used it to prime about 50 hemis with 60 weight oil in em.

The one with yellow tape is my home made piece. It's hollow tubing, solid will work too.