There really is no reason to be in 5th at 30 mph. I have only changed my sprocket to a 16t because I tend to stay at higher speeds when moving between gravel. Don't need a 16 and I might change back to stock, probably will. TBH, I see no reason to do 30 mph on gravel, fiddy + is more like it. So many folks are concerned about engine damage with "higher" rpms. As long as not near or in the red it won't matter one bit. Engines are designed for such "high" rpms.
FWIW, I have been forced, shift lever broke, to go 55-60 mph on gravel, in 2nd gear, for 30 miles at around 6k rpm. Absolutely no issues, got better mpg than my buddy with a dr 650 in 5th gear (we were both shocked), no overheating, etc....
I rode 300 miles of gravel yesterday and I like to keep the rpms up a bit. No sense lugging at all anywhere in gravel. I did tend to keep the mph at 55-70 though so no lugging anyway. Maybe you should jump in front of the wing and do 55 and show him how it is done.
Also, FWIW, lugging the engine IS putting stress on the motor, not "high" rpms!
'01 KLR 650, '09 Suzuki DL650, '08 Suzuki Bandit 1250, '16 CRF1000L, '09 KLX 250s