For a long time it was XOM which is a arrangement that XOM has with GM. I have read that Chrysler and SOPUS had signed a contract for SOPUS to supply Mopar branded fluid's but who knows now that Fiat has a controlling interest in Chrysler now.
My brother works for Chrysler. It is Pennzoil. They use platinum or ultra for the synthetic changes also. Gear lube, and transmission fluid is Valvoline. My bro also said, they carry Rotella triple protection. And everything comes in the big 55 gallon drums that says, premium conventional, platinum, or ultra.
Hope that helps,