Above link is for the Shell brocure-it looks like they use a variety of thickeners in this line of greases so you need to find out what exactly you have. I'm not familar with all of their formulations as I've only seen their PolyUrea based grease.
whether greases can be mixed depends both on the type of thickener used and the basefluid.
generally speaking greases with the same type of thickener and basefluid can be mixed, if they differ mixing may not be a good idea because either short term or long term effects may impart the correct behavior of the components involved.
the best advice is to contact the supplier of the grease you intend to use and ask him whether the new grease is compatible with the grease used hitherto.
mixing of greases is better avoided, especially in applications where a mixture may stay in place for a long time as in antifriction bearings or centralized grease systems with long lines.
if mixing takes place in journal bearings problems can usually be avoided by pumping so much grease into the bearing that all the "old" grease is pumped out and some "new" grease also.
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