What happens if you put motor oil in your car what will happen?

I put an 1/8 of a gallon of a mixture of gas and oil into the gas tank of my 2008 Honda Civic and drove about 3 miles to a gas station. When I got to the gas station I only filled up with about another 2 gallons and then drove home. The car drove fine and I noticed no difference. How bad is this for my engine? If I drained my gas tank right now and filled it with gas would it be fine?

If you used the gas/oil mixture for 2 cycle engines, you should be OK. There's no need to drain the tank. Just don't put straight diesel or straight motor oil in your gas tank like my cousin. BTW, I put a couple ounces of Marvel Mystery Oil in my gas every other fill up.

I'm assuming you ran out of gas and had to put weedeater gas in to make it to the station. As little as you put in I don't think it would be a big deal if you didn't already fill the tank all the way up I would that way it'll thin the oil more. It may smoke a little bit and I would suggest changing spark plugs if it starts sometime after a couple of tanks of gas in case it gets them a little dirty hope this helps

It will be like you have a bad valve seal, just burns the oil, lubes up the injectors helps some.

Marvel Mystery Oil is what works well in gas and oil, I use it in everything, even ad to lawn mower gas to keep the carb from fouling.

To protect the fueling system from contaminants, there's a fuel filter that holds rust, humidity, paint chips, other particles. I'm not sure how efficiently the fuel filter can prevent engine oil from seeping into the fuel system.

In all modern cars (MY 1986+) There are On-Board Diagnostics (OBD) designed to monitor and diagnose malfunctioning of fuel system and components including fuel injectors, fuel rail, evaporative emission system, oxygen sensors, fuel pump etc.

Now I'm not an OBD expert. But I know that in case of contaminants (Engine Oil for example!), fuel pump won't be able to maintain prescribed Fuel Rail Pressure. Fuel Injectors would be clogged and won't be flowing appropriately. This will cause misfires in multiples cylinders… All of which is detectable by the engine controller and the overall OBD system. The system will set a fault and depending on the severity of the situation, engine controller will prevent operation of the vehicle or just set the "Check Engine Light" on instrument cluster.

It did no damage at all and no, you do not have to drain the fuel tank. Rest easy you are fine. 1/8 gallon of oil/gasoline mixture had so little oil in it as to be of no consequence. Your biggest worry is damage to the fuel pump by running the car so low of fuel. The fuel pump is cooled by the gasoline, so running it low of fuel does the fuel pump no good. Always fill up when the tank reaches the 1/4 mark.

Don't sweat the small stuff!

Cat will be damaged with the 2 cycle mix.

I mix 2 ounces of motor oil with every tank refill on all my cars. It lubes the fuel pump and the upper piston ring.