Well, after about 200-300 rounds, the HW30S was still shooting sweetly, but it definitely smelled like the electric train I had as a kid -- you know, the one you put a drop of oil down the smokestack so it would puff white smoke as it chugged around the track. No detonation ever audible, no smoke visible, but my nose finally got tired of the smell of burning oil after every shot.
So I tore the rifle down this evening and though there was no visible damage to the piston seal or any apparent galling of the tube, there was more than a little thick, gunky blackend grease on the wrong side of the piston to be sure. After a careful clean up with LectraClean (trichloroethylene) degreaser, a bit of crosshatching, another cleaning, burnishing the tube with a bit of moly paste, a bit of polishing and de-burring a few sharp edges, insertion of a Maccari spring and guide, a careful lube job, and reassembly with LockTite and teflon tape in all the right places, re-cleaning the barrel with GooGone, and remounting and zeroing the scope, the HW30S is now shooting as sweetly as ever. And after the first 10 or 12 pellets (fingers crossed) NO MORE DIESEL SMELL!!! (But so much for the warranty, I guess). The HW30S action is somewhat different from the R9, but overall, a bit easier to work on. About three hours start to finish. Not too bad for a rookie. Goodnight.