34 34 u2032 which each define a respective flow distribution passage 36 36 u2032 each manifold 34 34 u2032 has a respective central longitudinal axis a a the peripheral edges of apertures 30 in abutting bosses 32 define a plurality of axially or spaced apart annular ridges 38 projecting into the flow distribution passages 36 36 u2032 the annular ridges 38 in turn define therebetween a plurality of axially or longitudinally spaced apart annular recesses or channels 40 which also form parts of the flow distribution passages 36 36 u2032 the bosses 32 form peripheral walls of the manifolds 34 34 u2032 the manifolds 34 34 u2032 are coupled to one another in heat exchanging relation such that in use upon a flow of heated oil being forced into one of the manifolds 34 34 u2032 a flow of cooled oil issues from the other of the manifolds 34 34 u2032 such coupling is effected in this embodiment by a plurality of heat exchange fluid flow passages or oil passages shown in phantom in fig2 and identified with reference numerals 42 formed by the plate pairs for greater clarity it should be understood that in this preferred embodiment the heat exchange flow passages 42 extend between the manifolds 34 34 u2032 encircling the central aperture 26 in a split flow configuration again other configurations are possible with continuing reference to fig2 the donut cooler 20 also comprises a top plate 44 and a bottom plate 46 the top plate 44 has ports 48 50 formed therethrough communicating with respective upper ends of manifolds 34 34 u2032.