For the most part, I don't worry so much about grease. The same spec grease can come in many different colors. The different types may not have the same qualities, but in my experience they don't conflict. Finally, the parts that require grease aren't terribly picky about what type, but just require enough grease to lubricate them. I'd run it and not worry.
I'm not a chemical engineer, just a dude who's done an utter crapload of wheel bearings and chassis lubrication in his career. Due diligence is to read the specs on both greases.
I called Mevotech's tech support again, and spoke with a different person, who gave me a bit more information regarding grease. He said they use a "semi synthetic, nanoceramic grease" and said it is litihium based.
I did an Internet search on nanoceramic grease, which I'd never heard of before, and search results mainly point to one product, DayLube "nanoceramic racing grease," manufactured by Dayton Progress Corp. of North Carolina. According to a person I spoke to who is a distributor of that product, that particular grease (DayLube) is tan in color, so I may
have found the grease that Mevotech uses. I've never seen any tan-colored grease before. In any case, both the guy at Mevotech, and the grease distributor, seemed to believe it should be okay to use a lithium-base grease with it (the red grease sold at my local auto parts store is lithium complex grease).
I don't know if flushing the existing grease out is an option; I imagine that would take some time, and the joint would have to be greased repeatedly over a short period of time. I am considering ordering some of the DayLube grease, but at $50 a tube, it isn't cheap. Leave it to Mevotech to use an oddball grease. Despite going down a rabbit hole on this matter, I'm still no closer to a definitive decision. Like you and others have said, I may just say 'screw it' and use the common red grease. It sucks to be OCD sometimes.