generally speaking grease is a better lubricant then oil for most roller bearing applications, unless you need the lubricant to also function as a "coolant", about 90% of roller bearings used are grease lubricated. however, the optimal choice of lubricant can be quite complicated, as can be seen in the excellent publications mentioned by Tmoose. careful analysis of the operating conditions the bearing will encounter is a prerequisite for making the right choice and often a "special" type of grease may be needed.
not each and every lubricant supplier is able to supply all possible grease types. some of them have specialised on products for specific types of application. generally speaking the marketed greases fall in two categories. on the one hand greases that are truly "multipurpose" in the sense that they combine corrosion protection, mechanical stability, water resistance, load carrying capacity and oxidation stability at such a level that most applications can satisfactory be lubricated. over the years the "application envelope" has been enlarged including higher temperatures, longer relubrication intervals, better mechanical stability and higher water resistance. lithium and lithium/calcium based greases and most complex greases are examples of this grease types and may account for over 75% of the volume of grease sold.
the second category are the "special greases" - products that excel in some particular areas, and usually are not optimally formulated where other requirements are also important. in this group you will encounter "low noise" greases, high temperature greases, greases with special cold flow properties, resistance to various chemicals etc. a lot of them are based on less familiar thickener types and synthetic base fluids. they can help out where "multi purpose " greases are not good enough - at a price. some special greases may cost up to 500 times as much per unit of weight then standard "multi purpose" greases!
