It looks like Dexron HP (LV) is different from Mercon LV.
Valvoline produces a synthetic Dexron-VI/Mercon LV fluid:
Valvoline™ DEXRON*-VI/MERCON* LV ATF is a full-synthetic transmission fluid formulated with advanced additive technology to meet and exceed the requirements of the General Motors* DEXRON*-VI specification. It is officially licensed and approved by GM* and is fully back-serviceable and can be...
www.valvoline.com
While Mobil's Dexron-VI fluid is not synthetic, but rather a synthetic blend:
Mobil™ DEXRON-VI ATF is a high-performance, synthetic blend automatic transmission fluid formulation designed to meet the stringent service fill requirements of GM’s DEXRON-VI specification.
www.mobil.com
I'd probably go with the Valvoline product, personally.