Can I Compost Motor Oil/Engine oil (from cars etc)?(From the Browse by type of thing, Household waste category | 1 comment - join the conversation)
No, do not add oil to your compost heap.
Motor oil/engine oil, and other mineral oils, can cause serious harm to the environment unless they are disposed of correctly. While there are certain bacteria that will break down oils, they are unlikely to be in great numbers in your compost heap and wouldn’t be able to break down the oil in a timely manner. The oil may also have a serious negative impact on the other friendly bacteria in your compost and drastically reduce the airflow within the heap (thus compromising the bacteria even more).
There is also the very serious contamination risk. Oil itself is bad enough but motor oil normally include a number of additives including detergents, oxidation inhibitors, rust inhibitors and PTFE (Teflon) particles to help with lubrication. You don’t want those getting into your food chain via your compost heap.
Finally, there is a combustion risk: oil vapors are highly flammable and can ignite from 100F/37C upwards. Well balanced efficient compost heaps can easily reach those sort of temperatures – in fact, some people claim that compost heaps should reach 140F/60C in their centre for maximum efficiency.
All in all, don’t do it.