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.
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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.
If you have some automatic transmission oil around the garage, especially the newer transmission oil, that would be better than a heavy oil on the chain. The heavier oil will pick up and hold all kinds of dirt from the road. Even some Marvel Mystery Oil ( remember that red stuff you could use for anything around the garage? ), would work well on the chain.
You can use gear oil if you want to, it will lubricate your chain. Some thinner motor lube will tend to spray off the chain when pedaling.
I did an experiment and used chainsaw chain oil on my own chain on my fixed gear bike. It did a good job of lubricating the chain. However, it tended to pick up a lot of dirt and grime. It was also much harder to totally clean off unless I used really harsh solvents (like paint thinner).
So, I'm going to recommend buying bike specific lube. It is easier to strip and re-apply using citrus degreasers or other products (simple green) and will not pick up gunk as quickly (especially dry lubes).
However, I'm not going to convince everybody. Chain maintenance is a religious issue and everyone has their own set of beliefs about how to do it properly.
Lot's of esoterica and info about chain maintiance here: http://sheldonbrown.com/chains.html
Short answer: yes, you can. I've been doing it for years and my chains are lasting very long compared to ancient times when I used other stuff. Only potential caveat is that it is not the cleanest oil you'll ever use, but neither are the similar, much more expensive "wet" oils in the market (namely FinishLine Green). One liter of gearbox will cost a few bucks and last for the rest of your life.
Hope this helps!