An adaptor plug for a spin-on fluid filter, in particular for a spin-on return filter, wherein the adaptor plug comprises a plug-in end which is designed to be plugged into a cavity in a flange of a hydraulic device in a fluid tight manner and a filter end which is designed to be connected to the spin-on filter in a fluid tight, detachable manner and at least one internal fluid channel connecting said plug-in end with said filter end, wherein the adaptor plug at its filter end comprises a threaded tube or bolt on which the spin-on filter can be screwed. By this design, the use of a filter set with or without an adaptor ring between the adaptor plug and the filter is enabled. By the adaptor plug, the spin-on filter can be applied to a variety of hydraulic devices, among others, with a ¾ 16 UNF cavity.