The 3D shock wave turbulent boundary layer interaction (SBLI) past a single fin on a flat plate features a quasi-conical symmetry apart from the inception region near the leading edge of the fin. However, the validity of this symmetry varies for different flow quantities and has not yet been verified for e.g. wall stream lines or wall shear stress. In this study, oil film interferometry (OFI) is used to measure the areal distribution of wall shear stress within the 3D interaction region induced by a single fin at Mach 3 and a fin angle of \(16^{\circ }\) and \(20^{\circ }\) . With improved and automatised evaluation procedures, the quasi-conical symmetry is demonstrated and used to further improve the OFI measurements. Additionally, 3D RANS simulations were performed for mutual verification and to quantify the disturbing effect of pressure gradients on the OFI results, which are usually neglected in OFI applications.