If it's thick and gooey, and you are using a fuel like SIG Champion with a Klotz and castor mix, it's probably just excess oil from the exhaust. If your usable fuel load is, or the typical amount of fuel you carry for the pattern (adjusted for temperature and air density) hasn't changed, it's excess oil, especially if you spike your fuel with extra castor. The goo on most of my models is tinted red like this, as I tend to use SIG Champion fuel. Unless you are un able to carry enough fuel for the pattern, I wouldn't worry about it. If it is as consistent as you say, I don't think it's broke and if it ain't broke, I don't fix it!
Type at you later,
Dan McEntee