Any material with greater density will sink in a fluid with less density. So the question is how to make the density of the oil greater or the density of the water lower.
First of all, you can choose an oil that has a higher density. Larger molecular weight compounds will generally have a higher density, and some oils may have suspended solids which will increase their density.
You may achieve success with some oils by using heat. As most materials heat up, their density decreases, but not necessarily at the same rate. If you choose an oil with a low coefficient of expansion, its density may stay close to its original value as the water's density decreases with heating. If you can get the water's density low enough before it boils, the oil will sink.
Rob Landolfi, Science Teacher
Whether something floats or sinks has to do with something called density. Density is the reason that a rock is heavier than a piece of styrofoam the same size. Oil's density is like the styrofoam, and the water's density is like the rock.
When talking about things floating, we use something called the "buoyant force" its easy to remember because buoy's float. The idea is that things that are less dense are pushed upwards and things that are more dense are pushed downwards. Since oil is like styrofoam, and less dense, it is pushed upwards while the water (more dense) is pushed downwards.
So in order to make your oil sink in water, you would have to increase its density until it was more dense that water. Unfortunately that is very difficult to do with liquids because you can't squish it like you can with styrofoam. With styrofoam you could just increase its density by smashing it together to make it more like the rock.
What you can do is mix the oil with something else. A solution (two or more things mixed together) of oil plus another ingredient would increase the density of the oil. This is the same reason that salt water is heavier than fresh water, the weight of the salt is added to that of the water, making it more dense.
Other than actually pushing the oil down with your hand or something, combining oil with another ingredient would be the only real way to make it sink in water.
Kenneth Rider, Undergraduate Student, UC Davis
'One thing I have learned in a long life: that all our science, measured against reality, is primitive and childlike - and yet it is the most precious thing we have.'