6 gallons seems a bit low...I have run this a bunch and usually change the o-ring about 75-100 flights. The Central Hobby Replacement O-ring is a different size than the hatori. The NMP header is 5/8" and the Hatori is 15mm. So the O-ring size is changed to fit the NMP header. You might have better luck. If you assume about 8 flights per gallon then you should be getting 8-9 gallons per o-ring. If you are not perhaps some better cooling may help.

Troy Newman
Team YS Performance