Conventional material aging and testing protocols involve exposing coupon samples to saturation in application fluid(s) at temperature and pressure conditions typically encountered during service, followed by mechanical testing at ambient conditions. This practice can generate misleading results for materials for which fluid ingress is rapidly reversible, most notably at elevated temperatures. A recently developed in situ punch–shear device has been successfully used to establish experimental correlations between the tensile properties (ASTM D638) and shear properties (ASTM D732) of Polyethylene of Raised Temperature (PERT) under dry conditions. It also enabled measurement of shear properties of select polymers while immersed (saturated) in fluids at elevated pressure and temperature. The present work extends the treatment to a suite of commercially available thermoplastic polymers spanning the commodity, engineering, and high-performance polymer grades with varying degrees of hygroscopicity. The objectives of this contribution are three-fold, namely: (i) assess the effect of sample preparation method on measured mechanical properties, (ii) compare the experimentally established correlations between shear and tensile tests for the different class of polymer grades before fluid exposure, and (iii) gauge reversibility of the measured tensile and shear properties after aging in deionized water to saturation at 95 °C. Results indicate that (i) the test coupon preparation method affects the tensile to shear correlation and must be standardized to enable systematic comparison of in situ properties, (ii) individual correlations segregate by polymer family, and (iii) conventional tensile testing after a saturation–dehydration cycle yields optimistic mechanical properties.