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you can add corn head grease or grease plus 600W mixture with out cleaning out old grease. It will mix with the old grease and be fine. All you are trying to do is get a thinner grease that will not form a cavity around the u-joint,but that will flow back to coat it with grease when it stops.
If you have a stock transmission with no seal on the rear bearing,the oil that leaks past it will thin the u-joint grease and keep it lubricated.
John