What could've caused this..

[maxreps]

buddy of mine has a 72 C10 and recently had problems he is sure were caused by his ex girlfriend putting something in his gas tank. She knows he loves his truck and I guess if she did it, she wanted to hit him where it hurt. Anyway, he had problems with it running at first like the carb was screwed up. He didn't even think about her at this point.. he pulled it, disassembled & cleaned it then put it back on. Well it started again soon after that and also a bit later started running really badly. After cleaning it again he pulled the plugs on a whim and they had some oil on them. After all this he started thinking that this was too coincidental and suspects she put something in his tank. But my question is, what the heck would/could cause gum/clog a carb and also cause piston ring problems? I've never heard of something like that and I think he may be wrong about her because unless she had help with the idea, I'd say she was not savvy enough to cause that much trouble.

BTW, this truck has ran flawlessly for almost 2 years that I know of....