This paper experimentally studies the effects of changing radial clearance C_{r} on the performance of a long (length-to-diameter ratio L/D = 0.65) smooth seal under mainly-air (wet-gas) conditions. The test fluid is a mixture of air and silicone oil. Tests are conducted with C_{r} = 0.188, 0.163, and 0.140 mm, inlet pressure P_{i} = 62.1 bars, exit pressure P_{e} = 31 bars, inlet liquid volume fraction LVF = 0%, 2%, 5%, and 8%, and shaft speed ω = 10, 15, and 20 krpm. The seal's complex dynamic stiffness coefficients H_{ij} are measured. The real parts of H_{ij} cannot be fitted by frequency-independent stiffness and virtual-mass coefficients. Therefore, frequency-dependent direct K_{Ω} and cross-coupled k_{Ω} stiffness coefficients are used. The imaginary parts of direct H_{ij} produce frequency-independent direct damping C. Test results show that, for all pure- and mainly-air conditions, decreasing *C*_{r} decreases (as expected) the leakage mass flow rate $m\u02d9$. Under mainly-air conditions, decreasing C_{r} decreases K_{Ω}. This outcome is contrary to the test results at pure-air conditions, where K_{Ω} increases as C_{r} decreases. Since an unstable centrifugal compressor rotor may precess at approximately 0.5ω, the effective damping C_{eff} at about 0.5ω is used as an indicator of the impact a seal would have on its associated compressor. For pure-air conditions, when Ω ≈ 0.5ω, decreasing C_{r} increases C_{eff} and makes the seal more stabilizing. This trend continues after the oil is added. A bulk-flow model developed by San Andrés (2011, “Rotordynamic Force Coefficients of Bubbly Mixture Annular Pressure Seals,” ASME J. Eng. Gas Turbines Power, **134**(2), p. 022503) produces predictions to compare with test results. $m\u02d9$ predictions correlate with measurements. Under pure-air conditions, the model correctly predicts the effects of changing C_{r} on K_{Ω} and the C_{eff} value near 0.5ω. After the oil is added, as C_{r} decreases, predicted K_{Ω} increases while measured K_{Ω} decreases. Also, for mainly-air cases and Ω ≈ 0.5ω, decreasing C_{r} does not discernibly change predicted C_{eff} but increases the measured value.