The louvered 16.7 L min −1 PM 10 inlet is commonly used in PM 10 and PM 2.5 FRM samplers or FEM monitors. Its sampling efficiency is influenced by particle bounce, re-entrainment, and overloading since the PM 10 inlet contains a PM 10 impactor with an uncoated impaction surface. In this study, a modified PM 10 (M-PM 10 ) inlet with an oil-soaked glass fiber filter substrate supported by an oil-soaked porous metal disc was developed to eliminate the particle bounce and overloading effects. The oiled M-PM 10 inlet and the traditional PM 10 inlets with and without grease coating were collocated at the field for long-term comparison tests. The results show that the traditional uncoated PM 10 inlet which is cleaned initially but not cleaned daily afterwards oversamples PM 10 concentration after one 24-h sampling day and has the high positive average sampling bias during 14 sampling days due to particle bounce and re-entrainment. The grease-coated PM 10 inlet without daily cleaning shows a better performance with a smaller sampling bias, but it still oversamples PM 10 concentrations after the first three 24-h sampling days and then undersamples after 10 sampling days due to particle bounce and overloading effects, respectively. In comparison, the M-PM 10 inlet shows a good performance with a small average sampling bias during 35 sampling days since vacuum oil wicks up through the deposit to eliminate particle bounce and overloading. It is suggested that the oiled M-PM 10 inlet can be used to replace the traditional EPA PM 10 inlet and for long-term sampling of over 1 month without the frequent maintenance need.