Small holes with light oil reacts at a slow shock speed the same as large holes with heavy oil. At high shock speeds the light oil will not flow linier through the holes but chaotic and this restricts the oil flow. This is the PACK you mentioned.
When you push the car down at the back you will not be able to tell if the car has small holes - thin oil or large holes - heavy oil but if you pick the car up and drop it then the car will not bottom out if small holes - thin oil but will with large holes - heavy oil.
Smooth tracks use small light
Bumpy track use large heavy
Dedicated RC Car Speed Site
Norfolk and Chance