Actually, I am thinking about it. [sm=wink.gif]
I'm thinking about the tuned pipe header setup I machined for an MVVS 40 GRRT a few years back that coupled metal-to-metal (ala motocross and kart racing pipes). And I'm thinking about all the tuned pipes I ran normal fashion with a silicone coupler. The only pipe setup that ever produced black goo was the GRRT and it all came out at the joint. And I'm thinking about the engines I've seen where their mufflers came loose in flight and all the black goo that oozed from the fretting surfaces between muffler and crankcase. And I'm thinking about the full scale airplanes that I work on for a living and all the black goo that comes from sheetmetal fretting together. (Ever notice the black streaks that trail back on a jetliner's flaps?) And I'm thinking about my O.S. 45 FSR that spun it's rod bushing at the big end and my friend's ST GS-45 that broke it's wrist pin, both engines spewing black goo out the exhaust at the time of failure. I've even seen wounded engines spew black goo when running 100% synthetic oil.