Restrictors are disclosed that include a body defining a Venturi tube having a throat defining a junction of a converging inlet cone to a diverging outlet cone along a longitudinal axis thereof, and with the converging inlet cone and the diverging outlet cone each defining an inner passageway that transitions as a hyperbolic or parabolic function toward the throat.

A combined oil filter and restrictor assembly is disclosed having an oil flow restrictor interposed between first and second filters so that, irrespective of the direction of oil flow through the restrictor, debris cannot enter a small diameter bore 32 in a body of the restrictor thereby reducing the likelihood of restrictor blockage. The combined oil filter and restrictor assembly includes a metal threaded member that enables the combined oil filter and restrictor assembly to be easily and quickly assembled to or removed from a component such as a cylinder head of a variable displacement engine. The combined oil filter and restrictor assembly can be used to provide a restricted flow of oil from oil supply passages to signal oil passages used for controlling the operation of deactivatable hydraulic lash adjusters so as to purge the signal oil passages when the deactivatable hydraulic lash adjusters are not deactivated.