An easy compound to use in oil/water marketing is fluorescein, a fluorescent dye. Fluorescein is cheap, easy to detect, and ideal for this sort of work. Micromolar and nanomolar quantities of fluorescein are detectable with a black light, millimolar very easy to detect with the eye in most cases, and easy with the black light as well. Really easy if any of you have one of these 405nm laser pen pointers that are out now for like $10. Not only do these lasers make great cat toys with some attenuation using a tape or film (watch direct illumination of kitties and reflections into eyes), their wavelength is good for exciting fluorescein.
If we're shooting for detection with a handheld black light (about $5) and some visual, and assuming a cooling system holds 2.5 gallons of water (10L), and fluorescein has a molecular weight of 388 g/mol, give or take, and we want a 1mM concentration....
Molarity = mols / volume
1 x 10-3 = n / 10 L
n = 1 x 10-2 mols
1 x 10-2 mols * 388 g/mol = 3.88g needed, so toss in 3-4 grams for the average car cooling system, or half that for an oil system and you should be good to go. You can get 500g of say 90% fluorescein for about $100 or so, about $0.25 a gram, from Fisher, VWR and others. We use fluorescein in my line of work and I generally have some in my trailer if anyone needs a dose for their car.