I'm a fan of using white lithium grease. I dont like the spray one, you end up using way too much and it leaves a wierd film. I like the tubs you can get for cheap from the auto stores. I dont see the point in overspending for a "gun" labeled grease when they are essentially the same thing.
I picked up a 1lb tub of white lithium grease for around $4-$5 dollars at checkers (now o'reillys). It might just last me the rest of my life. I like grease because it stays where I put it, and doesn't run.
I dont buy into the whole grease turning to mud thing. A gun will still work dirty as long as its lubed. Though I do belive grease starts to gun up in sub freeing temps. I wouldn't wanna use it on a .22lr in -10 degree weather. Lucky we live Hawaii though.
It should be noted that I dont JUST use grease. I use both oil and grease. I use oil where appropiate, and grease where appropiate. For instance, on my BCG for my ar, I use grease on the sides where there is major wear, but I use a thinner coat of oil (clp) over the rest of it. My M1a is more or less designed to run on grease better than regular lube also.
Something to note. If you over grease your gun, itll fly the excess off. Sometimes in your face. EYE PRO!
I've also used mobile one synthtic 10w-30... It works well. But I'll stick to my clp, since it works better as a cleaner.