The filter just needs the rubber gasket that comes already installed (wipe it with some oil before installing). The aluminum washer that you get from the dealer is to replace the crush washer on the oil drain plug. These washers can usually be reused a couple of times if you are sure you don't overtighten them when torquing down the drain plug. At worst, you will get a slight drip from teh plug if washer is worn out. Volvo uses aluminum washers, unlike most which are copper and take more torque and can be reused more.
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Mike Sullivan ('91 745 (171k), '93 965 (147k), '95 855T (60k), '98 V70XC (78k). Past Volvo's: '85 744 (256k), '86 245 (165k), '86 245 (195k), '88 745 (208k).