water repellency of a grease is a function of the base, not the oil.
grease is made up of two primary components: a base or thickening agent, which works as sort of a sponge; and the lubricating oil, sometimes called the carrier oil. the base holds the oil and actually sweats it out as temperature increases. the ability for the base to reabsorb the oil when it cools is called reversability. you may have noticed a puddle of oil under your grease gun. this is carrier oil that "escaped" from the base and was not reabsorbed.
the carrier oil can be organic (i.e., from dead dinosaurs and plants) or synthetic (made by polymerization of hydrocarbons). when manufacturers say that their grease is "synthetic" they are talking about the oil. unfortunately, the quality of the oil has only very little to do with the quality of the grease. more important is the base, which holds the oil on the parts and (hopefully) retards water ingress.
(note: oftentimes additives are included in the grease mix, for example moly or extreme pressure additives like zinc. but these additives have nothing to do with water repelency, which is the poster's question.)
typical bases include lithium, aluminum, clay, and calcium. all have their advantages and disadvantages. aluminum base greases, by far, have the very best water repellency. however, high quality lithium-complex bases also have good water repellency and are suitable for many automotive applications. you should not be using low cost hardware store greases for critical drivetrain applications. get good grease.
for the absolute best water repellency, use an aluminum base grease like Bel Ray Waterproof Grease or a high quality lithium complex like STA-LUBE STA-PLEX Premium Red Grease.
for more info, you may want to read
http://www.bobistheoilguy.com/GREASE/index.htm
and
http://theoildrop.server101.com/ubb/ultimatebb.php?ubb=get_topic;f=4;t=000221#000000
http://theoildrop.server101.com/cgi/ultimatebb.cgi?ubb=get_topic;f=4;t=000223#000000
here's an easy-to-do test for a "waterproof" grease...
pull on a pair of latex gloves. put a dab of your favorite grease in the palm of one hand. now run some water into the grease. use your other hand to swirl the grease around, and keep running water through the mix. if the grease turns whiter or milky or otherwise appears thinned or affected by the water, it ain't waterproof. only the best lithium complex base and most aluminum base greases will pass this trivial test. by and large, aluminum based greases have the best water resistance. bel-ray "waterproof grease" is an example of a good quality aluminum base grease.
jim aka the wrooster
