So you see a dark patch appear under you car engine after you have had an Oil Change.

During an Oil Change, two items are removed from the engine. One is the Oil Filter, which is replaced with a new Oil Filter. The other is the Drain Plug that goes into the Oil Pan. This Drain Plug is put back into the Oil Pan after all the old oil has been drained out. Initially a few hours after an Oil Change you may have a little bit of oil drip as some engines have the Oil Filter in such a way that Oil will spill onto the engine as the filter is being changed. After a few hours, you should no longer have any sign of oil dripping.

First of all you need to determine if the Oil Leak is coming from the Drain Plug or the Oil Filter. See below for descriptions of the Drain Plug and Oil Filter.

At the bottom of the engine is the Oil Pan. Into the side or bottom of this Oil Pan is a bolt called the Drain Plug.

Common reasons for a leak here are:

Gasket part of the Drain Plug is worn or Dirty and so the Drain Plug no longer has a tight seal on the Oil Pan. To fix, drain oil out of oil pan into container for recycling. Clean Drain Plug and part of Oil Pan it goes against and/or purchase new Drain Plug. Put Drain Plug back in Oil Pan and refill engine with correct amount of oil and oil type. Note that not all Drain Plugs will have a gasket as some just rely on the plug to be tight against the Oil Pan. Any dirt that gets between the Drain Plug and Oil Pan can cause a gap to be left that allows to oil leak out.

Drain Plug was not tightened all the way so it does not have a tight seal on the Oil Pan. To fix, take wrench that matches size of Drain Plug hex end and tighten up. Note that dirt may be between the Drain Plug and Oil Pan preventing it from being tightened up securely against the Oil Pan. If this is the case then the Drain Plug will need to be removed and cleaned as in the previous step.

Drain Plug was cross threaded when put back on the Oil Pan. This is where the Drain Plug is not put back in correctly and the mechanic forces it on using a wrench and in the process destroys the thread of the Oil Pan. This is very common on cars that have an aluminum Oil Pan. Only two ways to fix are either to have the Drain Pan replaced or buy an expanding rubber plug that replaces the original Drain Plug. Both fixes require the Oil to be drained out of the engine. With the rubber plug Drain Plug, it fits in the Drain Plug hole and a bolt compresses it to make it expand and seal the hole. Drain Pan replacement can be difficult if parts of the frame or suspension of the car get in the way of removing it.