Answer

We have had no reports of rusting or corrosion caused by using coloured water in the Boyle's law apparatus - and they have been supplied without oil for a few years now. In fact the manufacturer recommends that coloured water can be used instead of oil. When questioned why the recommendation has changed, they say that oil has traditionally been used due to the risk of some of the internal parts rusting. However, they now use non-rusting parts in the manufacture so it is no longer necessary to use oil. In fact they claim it works better with water!

If you would rather use oil it will still be perfectly alright to do so - any light (i.e. thin or less viscous) engine oil will be fine to use.  2-stroke engine oil is also suitable and has some colourant in it, which will make it more visible.