Although one should make it a practice to clean out any old grease before regreasing or using a different grease type, your statement is not entirely true:



Polyurea shear stable (di-urea) grease is compatible with other types of grease. Polyurea (di-urea), Mobil Polyrex EM or Chevron SRI, is factory fill/used in electrical motors and ceramic bearings...SKF, NTN, Timken, etc are just a few manufacturers that specify polyurea for electrical motor bearings. For other applications, a lithium complex grease is factory fill, like Shell Alvania 3. To me, Park grease is just repackaged Mobil Polyrex EM.

Outside of Park Tools grease being polyurea, does anybody know the msds of or what's in Campagnolo Lubrificante Grasso or Kristal SR7 Grease, Shimano factory fill grease, the grease used by the various bottom bracket and hub bearing companies, etc etc? Seems like there are more than a few posts/opinions that polyurea is the only grease that is formulated/recommended for bicycle use...to a point where they should become sales reps for Park Tools grease....lol.

While some greases last longer than others and or excel in one area or another, I have yet to read or hear about unusually high wear or catastrophic bearing failure directly attributed to using automotive greases.