Flat Plate Solar Hot Water using motor oil ???

I am planning on installing a solar hot water system. I want to use oil or some other liquid to runthrough the collector on the roof and then run it through a heat exchager to heat the water. I have seen systems with antifreeze, mineral oil, peanut oil, and some very expensive commercial chemicals that absorb and store heat extremely well but cost over $700 per gallon, (Doupont). What I am looking for is a liquid that will have a long life before it has to be removed and replace. Motor oil and cooking oil will only last about one year before it starts to break down. Antifreeze, about six months. Does anyone know of something that last longer and may also be environmentally friendly? Also anyone with a good heat exchanger design I would be very interested.

Thx, captarcandspark