A trumpet player friend of mine had recommended Hetman to me, and I was using it when I first bought my Mack Brass horn. I, too, had some gunk buildup. I switched to Blue Juice, and the gunk decreased to almost nothing. A few months ago, I tried Yamaha valve oil, and there's not only been no gunk buildup, but I've been able to go longer between re-oiling. YMMV.
Dean L. Surkin
Mack Brass MACK-EU1150S, BB1 mouthpiece
Bach 36B trombone; Bach 6.5AL and Faxx 7C mouthpieces (pBone on loan to granddaughter)
Steinway 1902 Model A, restored by AC Pianocraft in 1988; Kawai MP8, Yamaha KX-76
See my avatar: Jazz (the black cockapoo; RIP) and Delilah (the cavapoo) keep me company while practicing