Dear Sillyman,
I somewhat disagree with your statement "So i conclude the slug force has an effect on the natural frequency of the system...".
1. Slug force will not affect the natural frequency of the piping system. The support configuration will.
2. Check out and compare the nonlinear supports in static analysis (gaps, liftoff) in both the cases. Dynamic analysis will not consider non linear effects, ie. In dynamic analysis the inactive support will be removed (support that is lifted or gap that is not closed).

Use lumped mass in case:
1. A mass that you have not included in static case.
2. A concentrated load that will have major impact on the dynamic output.
3. Refer Caesar technical reference manual for detail.

Regards.