A wide variety of resistive and field dependent behaviors have been previously observed in both doped and non-doped Highly Oriented Pyrolytic Graphite (HOPG). We find HOPG samples to vary significantly in their temperature dependent resistances, even between portions taken from the same sample, yet they exhibit consistent non-saturating magnetoresistance (MR). The scaling behavior of the MR is shown to be characteristic of a model based on the Hall effect in granular materials. In addition to the large, field-linear MR, all samples exhibit Shubnikov-de Haas (SdH) oscillations. Additional samples were doped via ion-implantation by boron and phosphorous, but show no signs of superconductivity nor any systematic change in their magnetoresistive behavior. Analysis of the SdH data gives a 2D carrier density in agreement with previous results, and a large mean-free path relative to crystallite size, even in samples with thin ion-implanted surface layers.