Saw this at the Bed bath and beyond today:

http://www.bedbathandbeyond.com/product.asp?order_num=-1&SKU=11720307&RN=210

I didn't get a chance to look really closely at it in the store.  It did
say that it should not be used above 400 degrees F.  It may have been
talking more about the wire going to the sensor than the sensor itself.  It
also had some sort of alarm feature.  You could mount the sensor down into
the oil, maybe through the filler area?    You would just need to extend
the wire to the front of your van.  You could set the alarm to let you know
when your engine was completely cooked!

Edward