Can I use gear oil on my chain? I don't have any chain oil right now, but I have some gear oil left over from working on my car.  Would it be an effective chain lubricant?  It's listed as 80W90, but reading the Wikipedia article suggests that that's not equivalent to motor oil viscosity ratings.
Short answer: No, you shouldn't. Heavy oils attract too much dirt, grit and grime which will damage the useful life of your chain.

You need a light lubricant which will wick it's way into the internals of the chain, rather than simply coating the external portion of the chain.

A good Teflon carrier lube like the Finish Line Teflon works well, lasts a good while and doesn't pick up dirt when properly applied. Using something like that will extend the useful life of your chain.

Gear oil is too thick, and designed for a higher heat production than you get on a pedal driven bicycle. It relies on engine heat, and the gears moving at high speed to produce enough friction to reduce the viscosity of the oil enough to be thin enough to flow as a good lubricant. 

It would be better than nothing, but hardly a good choice.