Here's an article. Yeah it's a custom blend.
Quote
https://www.motortrend.com/news/the-science-and-smoke-behind-nascars-mobil-1-motor-oil-229627/
Blenders get their information from formulators, the folks who determine how much of each component needs to be mixed. For (Tony) Stewart's car, formulators create a special blend designed to reduce friction and increase engine power with the goal of improving lap times.