2000 Jeep Cherokee

Foamy yellowing material on the dipstick and under the oil fill cap are a good sign of water or at least moisture getting into the oil system. Has it been using water? I check for a head gasket leak two ways. One is use a pressure tester on the radiater and pump it up to what the cap pressure says usually between 12 and 16 pounds. Run the engine and moniter the pressure reading. If a head gasket is leaking right into the water jacket it will sometimes show up here by raising the pressure on the gauge. One sure way to test it (but you probably dont have the tool) They make a head leak tester that uses chemicals. You suck up some of the radiater fluid into the tester and if there are exhaust gases present it will change color. The first test is not that accurate but can be used, keep an eye on your water level if it has a head leak it will be getting lower. Also when you first very first start up the vehicle if you see white smoke just for a bit that is a good sign you have are burning antifreeze but you probably wont see this if its not stored inside. Hope this helps some. Curt