The rubber coating on the originals is thin on top but rather thick on the bottom, sealing side. It squashes out after a while and the nuts get loose. You may be right about brass being too hard. I tried some brass ones from McMaster-Carr and they leaked. So now I stick with the copper ones, they've always worked for me.
That set from Hoos is a simple solution I guess, if you're a financial idiot, lol. Why pay near $20 for 8 washers when you only need 4??? And even if they were sold as 4 packs, why pay $9 when you can do the same job for about $1.25? And I never understood why some insist on changing all 8 washers. The inner 4 don't need to be seal washers, there's no oil flowing down their studs. Some say because they want all the washers to match. Huh??? Those inner 4 are totally covered by the frame, top motor mounts, and gas tank - YOU CAN'T SEE THEM. OK, rant over, lol.