I've put together a few different drain fittings, and usually seen nylon tapered washers that seal the slip joint. But this time, when I bought a sink tailpiece extension, it came with a square-cut rubber washer instead. After installing it, the connection leaks very slowly. What is the intended use of the rubber vs nylon washers? This is the exact piece that I bought, which included the rubber: Form-N-Fit 1-1/4 in. PVC Flexible Tailpiece Extension
In my experience the flat rubber washer is used only where the tailpiece attaches to the strainer. The tapered nylon washers are used when connecting slip together sections of pipe.