Use the oil recommended by the equipment manufacturer. Most systems use AW32 (light viscosity) or AW46 (medium viscosity) hydraulic oil. Cheap hydraulic oil is not cheap when your system overheats and your pump is ruined!
The inner diameter (I.D.), outer diameter (O.D.), and height are the most common ways to measure a seal. It’s best to take these measurements from the cavity where the seal is located and the shaft which goes through it.