You can use any mineral oil, many people use "3-in-One" oil, which is available in most drug store and virtually all hardware stores.
So there's your answer, but here's my recommendation. Don't use any oil at all on your brush(es). Just clean them very well. That's all you really have to do, the oil is just used to make up for poor cleaning. For cleaning use Mineral Spirits if you are using a pinstriping paint like One Shot, but use L-thinner if you are using a urethane paint, like HOK pinstriping paints. I will tell you that I have never used any oil at all on my brushes and never will. I simply refuse to introduce potential contaminants to my paint jobs, and I always burry striping under the clear.