Motor oil seems at first glance to be a natural (and cheap) way to lube your chain. The fact of the matter is this, your chain will rust from the inside out if you lube with 10w-40 (or any other motor oil for that matter).
What? How can this be? Motor oil is thick, very thick. So thick in fact that it is unable to penetrate into the small parts of the chain. As a result, rust will start to form, and spread, in the inside of the chain. The outside of your chain will be a well lubricated chain. In about a month, your chain will be frozen solid from the rust.
We always get a good chuckle when a customer comes in because the chain is not "moving". We look down and see a black mess informing the customer that the motor oil they used caused the chain to rust solid.
Once an estimate to repair the bicycle is reviewed we always hear "I just thought that oil was a natural way to lube my chain".
Well, you will have problems LONG before your chain rusts. Motor oil is an EXTREMELY dirty product. After you pedal just a couple of miles, your chain will be a filthy, black, globby mess. Your pulley wheels will be caked and sticky (causing your chain to skip), and your rear wheel’s braking surface will be perfectly lubricated with a slippery black mess.
Getting your bike back to a safe condition after you use motor oil will cost you a lot of time and money. You’ll need a new chain and possibly a new rear derailleur at the very least. Plan on new rear brake pads as well as they will be soak with oil. Add in a hefty labor expense as well for cleaning oil from wheels, pedals, front chain rings, frame and your clothing.
So, please, whatever you do, DON’T USE MOTOR OIL TO LUBRICATE YOUR CHAIN. Invest $5 in a good, wax based BICYCLE lube for your chain. It will be the best $5 you’ll ever spend on your bike.