The Inside-Out Ceramic Turbine (ICT) is a promising microturbine for aeronautics applications. To increase the cycle efficiency and reduce fuel consumption, microturbines must operate under the recuperated Brayton cycle. The addition of a heat exchanger (HEx) increases the weight of the engine and cancels out any fuel savings as compared to a non-recuperated turbine. For this reason, the requirements applied to HEx for aeronautic gas turbines are: low weight and volume, high effectiveness with a low pressure drop, capabilities to endure high pressure and temperature, and low cost. The Laser powder bed fusion (LPBF) opens a new design space for geometries that cannot be realized by conventional methods, but induces high surface roughness. This paper presents experimental and analytical studies of the influence of surface roughness on the performances of a 3D-printed, counterflow, mini-channel HEx, with 1% of the total mass flow rate of the ICT. Results showed that the friction and heat transfer are both increased in the the regime typically defined as laminar compared to the analytical results. The experimental results are in agreement with a 1D heat transfer model when using correlations for high-roughness values from the literature. LPBF is a promising method to manufacture gas turbine parts, but it is crucial to model and incorporate its manufacturing capacities in terms of precision and surface finish to enhance HEx heat transfer and potentially reduce mass.