I've gotten 8+ on a well maintained .15 touring engine with NO loss of compression. I still have that engine in my garage and it purs like a female pee pee cat..hehe. Regardless of proper matinence, some engines are overall better quality then others. I wouldn't expect to be able to run more then 4 gallons or so from an aggresively ported racing engine but it's possible.
Use afterrun oil or ATF fluid (the idea is that it burns off) after EVERY day or bashing or racing. Clean your air filter (properly) as much as it needs it and ALWAYS use proper air filter oil when running on sand. The smallest particles of dirt can seriously shorten engine life.
EDIT - Also, always lube up your engine bearing/s with afterrun oil or a quality lube. I've lost engine in the past to bearing breakage.
Peace