Each blower and motor manufacturer usually recommends a #2 consistency(thickness) grease that will "stick" to the bearing surfaces without being "thrown" off. For example, a white general purpose axle grease is not recommended. By the same token, some graphite impregnated greases may be too thick and "sticky" resulting in the grease holding in the heat generated by the high speed of the bearing. Manufacturer's go to great lengths to be sure the grease they recommend is suitable to providing extended life of their bearings.

The amount of grease in a "sealed for life" bearing is very small--probably less than a tablespoon. Since bearing life is generally designed for at least 5 years, application engineers apparently figure it doesn't require much grease to keep a bearing lubricated. So why do you have to keep adding grease to all of your equipment bearings? Because they are not in a controlled environment. So many variables: heat, vibration, shaft alignment, foundation, and operation of the equipment can affect the bearing life.

However, one of the most common causes of bearing failure is contamination. Hot humid days followed by cool nights will condense the moisture in the air to leave water on any exposed surface of the steel bearing-then rust. The moisture can also mix with the grease as it spins to dilute and compromise the lubrication. If the air can enter the bearing housing cavity, so can dust, dirt, and other chemicals that can breakdown the grease.

As most mechanics know, not all greases are compatible with each other. Mix an oil based grease with a synthetic based grease and it will turn black and runny-like water. This is no longer an effective lubricant. Landfill gas applications are notorious for bearing failures due to contamination from dirty gas and moisture entering the bearing cavity.

Following the manufacturer's recommendations for type of grease and frequency of adding grease is usually the safest way to maintain the lubrication. I usually recommend 2-3 shots quarterly for units operating 24-7, and then a visual check of the grease condition once or twice a year, depending on the application, and type of operation. Over-greasing is as much a problem as under-greasing. You need to just add "enough".  

Any way you look at it, you have to do the preventative maintenance. Invest in PM, it will save you money!