dps650rider
XS650 Addict
Been using these for my valve covers with good results. In practice I find that it is best to replace them when I have a cover off but at this price who cares.
https://www.grainger.com/product/GRAINGER-APPROVED-Round-Medium-Hard-Buna-N-O-41KX45
https://www.grainger.com/product/GRAINGER-APPROVED-Round-Medium-Hard-Buna-N-O-41KX45