From a Tundra thread, by a lubrication engineer:
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Moly additized greases are excellent lubricants,however, should only be used in limited applications as the moly can actually cause early failures in most uses. Never, ever use a moly additized grease in a wheel bearing or universal joint application. Moly additized greases are primarily indicated only where extreme pressure sliding action occurs. When Moly is used in non-friction bearings the moly causes accelerated surface degradation (frosted mug appearance) of the rolling surfaces and premature bearing/race failure. The moly is actually harder than the hardened bearing surface and does not crush when the bearing rolls over it. In universal joints, in addition to surface failure, the moly centrifuges to the extremes of the bearing, blocking oil flow, causing accelerated bearing end wear.
If a single grease is being considered a non-moly grease is the safest primary recommendation. The use of both moly/non-moly specific for sliding/bearings application is ideal.
I currently have 5 failed universal joints sitting on my lab table with very low time; all were regularly greased with a moly additized grease and each failed due to the use of the moly grease.
George Morrison
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I know what the manual says with respect to U-Joint grease. My statements are from a lube engineer's perspective (along with 6 inches of scar tissue on derrier from having used Moly in U-joints before this was known!). CV joints, however, are another story.
Purging the U-joint will work fine; it is continued use of the moly grease over time which causes the problems..
One can never go wrong using a high quality non-moly grease in every applilcation (except CV joints), however the same is not true of moly greases. There are so few real applications and need for moly additized greases and so many potential problems as a result, moly greases are like a curse for us lube engineers..
George Morrison, STLE CLS
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We should never, ever use a Moly additized grease for wheel bearings or any non-friction type bearing. Moly is superb for plain bearings (anything without ball or roller bearings) such as are used in back-hoes, bulldozers, etc. In these cases, severe impact loading can exceed the film strength of the base oil and the moly can act as a fail-safe lubricant and work superbly. However, in non-friction bearings we are relying on the oil film to do the lubrication. Grease is composed of 80 to 90% oil; we are lubricating with oil, just as tho you were oiling the bearing with an oil can. The remainder 10% is sponge which holds the oil in suspension for use when pressure calls for its need.
Color has no bearing on grease performance EXCEPT for one area: never use a black grease in a non friction bearing/wheel bearing/universal joint. Period.. Use yellow, purple, pink, red, whatever, just no moly. Moly will give a steel grey appearance to a grease. There are black greases which contain no moly and they are fine, of course.
George Morrison, STLE CLS
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YMMV, of course