We report on dc magnetization measurements of high-quality single crystals of the title organic super-conductor. The temperature dependence of the magnetic penetration depth is estimated from the slope of the linear M(lnH) dependence in the London regime. An excellent agreement with the BCS theory for a weak-coupling superconductor in the clean limit is found. An attempt is made to relate the mixed-state-derived penetration depth, (T0)=53520 mm, to normal-state Fermi-surface parameters. The strongly correlated heavy carriers appear to be of crucial importance to explain superconductivity in this material.