Aftermarket oil filler cap rubber gaskets from reputable manufacturers like Reinz and Elring are typically under $1 and perfectly decent in my experience. Reinz is a well known supplier to Volvo. At that price I usually buy a few at a time. They only take a moment to change as soon as the old one starts to become hard and brittle.

Probably not a bad idea to order one or two cheap copper washers for the oil pan drain plug at the same time under the same shipping charge. After a couple of uses they also become hard and prone to weep. Or you can reuse them, using heat to anneal and soften the copper occasionally.
--
Dave -still with 940's, prev 740/240/140/120 You'd think I'd have learned by now