Just got a follow-up note from Valvoline:
Our Valvoline SynPower Grease is a full Synthetic Lithium Complex Grease; while our Valvoline Multi-Purpose Grease is a Conventional Lithium Complex Grease. Both are 100% compatible and may be mixed with one another with no reaction or problems. Multi-Purpose Grease is engineered for maximum performance and protection in suspension systems, wheel bearings, steering linkages and many more application.
Short answer, totally compatible, both have Lithium Complex base. If you want Moly, use SynPower. Don't want or need it, use Multi. Also slightly higher temp ratings on SynPower.
I have to laugh, Multi is "maximum performance and protection" yet SynPower is somehow better at twice the price. That's just marketing falling over themselves to find enough superlatives. Valvoline is not the only one trying to find ways to describe higher end products, when they've already used up all the superlatives on the regular products. For my modest needs, either will do.