Willym,
In the last several years I've serviced several trailer wheel bearings. In all cases the lube used was of a lower viscosity than I used when servicing cars and trucks. As stated I believe that the regular bearing lube is thicker and would not under all conditions travel through the small hole drilled into the axle which is there to permit the grease access to the inner bearing. Some times things like dirt, moisture, clumps of stuff get in the way of the grease. Even when greasing kingpins or tie rods you couldn't get the grease to enter. To reduce the chances of blocked passages the lube they recommend is of a lower viscosity. If you used say Amzoil synthetic grease it would be very difficult to get the grease to flow through the hole all the way back to the inner bearing. It's a better grease by far but if it does not get there it's not much good. So they recommend the lower viscosity grease.
I remember trying to grease some vehicles with an air powered grease gun. When that zerk is plugged, or the kingpin has not been greased for a long time, you have to really press hard on the grease gun and even then the grease will squirt out without getting past the plugged zerk fitting or into the bushing or ball joint.
Again many will not agree but just my thoughts on why and what I do.
TeJay
