Loosing Coolant

2002, Grand Jeep Cherokee, 2x2, 37,000 miles. My son drove the Jeep from Orlando, FL to Miami, FL. The following day my Dad changed the oil. To his much surprise the oil was gulk. When the Jeep started it made a clacking noise. My son stated that he had not heard the noise when driving the evening before. The Jeep was taken to the Dealer. The Dealer indicated that we must of added a deriative (STP) to the oil and this is what caused the gooie appearance. Note: the coolant in the reservoir was low. Supposedly, the Dealer fixed the Jeep. Ten days later the engine light turned on. The coolant reservior was low again. The reservior was filled up by another Dealer and the engine light turned off. Seven days later the engine light turned on again. Again, the coolant reservior was low. The car was returned to the original Dealer. Now they are claiming that the engine light is not on due to the low coolant in the reservior but due to problems with the transmission. The Jeep had previously been taken to this Dealership for transmission problems and they never found the problem. When the Jeep accelerated, sometimes, it would make a jerk type motion to change gears rather than a smooth transmission. This problem was fixed by the other Dealer. With the above brief description what do you think is happening to the coolant in the reservior? Is it possible for the engine light to turn on for the transmission or is it on due to the low coolant in the reservior? Please Help! : ? :