Lithium-conducting garnets are a promising solid state electrolyte in the cubic phase for lithium (Li) batteries because of their stability with lithium, wide electrochemical stability window, and high conductivity (~1 mS cm^-1). Aluminum (Al) is a widely studied dopant due to its ability to stabilize the cubic phase because of the occurrence of unintentional doping from alumina crucibles used during synthesis. The quantity of aluminum in the material is critically important to sintering conditions and performance in both the bulk and at the grain boundaries. Al can diffuse into the structure of garnets and dope the Li site while removing three total Li for charge balance. Another possibility is the Al reacts with excess lithium or lithium that has left the structure during high temperature processing. This allows for a liquid phase to form, enabling better densification during sintering. In this work, the aluminum content in Li_6.75La_2.75Ca_.25Zr_1.5Nb_.5O₁₂ was varied and calcined in magnesia crucibles to limit Al contamination. One weight percent of alumina (0.1 moles) was added before the solid state reaction, producing a conductivity of 2.96 x 10^-4 S cm^-1 after sintering at 1100°C. Specifically in the sample with 0.1 Al added when sintered at 1100°C for 12 hours, the Al contributed to an increase in grain boundary conductivity to 1.96x10^-3 S cm^-1 and a decrease in total activation energy to 0.35 eV versus samples without any added aluminum. The varying Al content showed little variance in bulk conductivity over several sintering conditions, while showing a wide divergence of 2.72x10^-3 S cm^-1, in grain boundary conductivity. Possible explanations are discussed with structural, chemical, and EIS measurements detailed.