We report on a systematic NMR decoration study of the field inhomogeneities in superconducting cuprates YBa2Cu3Ox (6.5 <x < 7) in the vortex state. The penetration depth as a function of temperature, lambda(T), is determined from the temperature dependence of the additional NMR line broadening Delta w of a thin oil layer covering the superconducting grains. This additional line broadening is caused by the field modulation in the mixed state at the surface of the grains. The lambda(T) behavior can be well characterized by the simple power low lambda(2)(0)/lambda(2)(T) = 1 - (T/T-c)(n). We found that the low temperature value lambda(0) is decreased and the power n is reduced as the oxygen content is lowered. For a decreasing oxygen content x we see a systematic reduction of the power n. The evolution of lambda(0) and n with doping is discussed in the framework of the d-wave pairing scenario and the strong electron-phonon coupling in layered superconductors. (C) 1998 Elsevier Science B.V. All rights reserved.