Rebuilt engine, bent push rods?

So after the engine builder took the head off, he discovered somethin really weird...the entire interior of the engine is coated in some strange substance similar to some really really bad gas. On most of the interior of the engine it has hardened to almost a glass-like consistency. Neither of us can figure out what it is or where it came from...got any ideas?
Thanks.
Oh, the reason he took the head off in the 1st place is because we noticed that one of the valves was sticking intermittently.